Debt - Additional Information (Detail)				3 Months Ended
Debt - Additional Information (Detail)	May 18, 2020 USD ($)	Aug. 02, 2019 USD ($)	May 02, 2017 USD ($)	Mar. 31, 2021 USD ($) d $ / shares shares	Dec. 31, 2020 USD ($)	Aug. 04, 2015 USD ($)
Debt Instrument [Line Items]
Long-term debt, net				$ 39,758,000	$ 44,114,000
Interest on final payment				6.75%
Carrying value of term loan				$ 53,100,000
Current portion of long-term debt				$ 18,333,000	$ 16,806,000
Repaid In Year One [Member]
Debt Instrument [Line Items]
Debt instrument interest rate				3.00%
Repaid In Year Two [Member]
Debt Instrument [Line Items]
Debt instrument interest rate				2.00%
Repaid In Year Three [Member]
Debt Instrument [Line Items]
Debt instrument interest rate				1.00%
Repaid In Year Four And Thereafter [Member]
Debt Instrument [Line Items]
Debt instrument interest rate				0.00%
2015 Term Loan [Member]
Debt Instrument [Line Items]
Repayments of lines of credit				$ 7,700,000
Carrying value of term loan				53,091,000
Liability component of Convertible Notes recorded as long-term debt				60,659,000
Mid Cap Financial Trust [Member] \| 2015 Term Loan [Member]
Debt Instrument [Line Items]
Term loan, maximum borrowings						$ 30,000,000.0
Long-term debt, net				$ 34,800,000
Silicon Valley Bank [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Term loan, maximum borrowings		$ 20,000,000.0
Debt Instrument Maturity Date				Jan. 01, 2024
Minimum liquidity threshold				$ 80,000,000.0
Minimum liquidity threshold, percentage				100.00%
Debt instrument interest rate description				Borrowings under the revolving credit facility accrue interest monthly at a floating interest rate equal to the greater of the prime rate or 5.50% per annum.
Unused portion of revolving line, percentage				0.25%
Drew down amount from credit facilities		20,000,000.0
Line of credit facility, fee description				unused revolving line facility fee equal to 0.25% per annum of the average unused portion of the revolving line, multiplied by the difference between the total amount available to be borrowed (the “Revolving Commitment Amount”) and the greater of the average outstanding revolver balance and 25% of the Revolving Commitment Amount
Line of credit facility, minimum interest calculation as a percentage of revolving commitment amount				25.00%
Revolving credit facility and any related fees, description				Beginning on January 1, 2020, if the interest payment on the revolving credit facility is less than the amount of interest that would have been payable had the Company borrowed 25% of the Revolving Commitment Amount, then the Company will be required to pay the difference.
Silicon Valley Bank [Member] \| Revolving Credit Facility [Member] \| Repaid In Year One [Member]
Debt Instrument [Line Items]
Line of credit facility, termination fees, percentage				2.00%
Silicon Valley Bank [Member] \| Revolving Credit Facility [Member] \| Repaid In Year Two [Member]
Debt Instrument [Line Items]
Line of credit facility, termination fees, percentage				1.00%
Silicon Valley Bank [Member] \| Revolving Credit Facility [Member] \| Repaid In Year Three And Thereafter [Member]
Debt Instrument [Line Items]
Line of credit facility, termination fees, percentage				0.00%
Silicon Valley Bank [Member] \| 2015 Term Loan [Member]
Debt Instrument [Line Items]
Long-term debt, net				$ 40,000,000.0
Silicon Valley Bank [Member] \| 2019 Term Loan [Member]
Debt Instrument [Line Items]
Term loan, principal amount		$ 40,000,000.0
Debt Instrument Maturity Date				Jan. 01, 2024
Amount borrowed	$ 15,000,000.0
Debt instrument interest rate description				Term loan borrowings under the credit facility accrue interest monthly at a floating interest rate equal to the greater of the prime rate plus 1.5% or 6.5% per annum
Term loan, first periodic payment date		Feb. 01, 2021
Interest on final payment				6.75%
Term loan, payment description				interest-only period of 18 months, principal is due in 36 equal monthly installments
Silicon Valley Bank [Member] \| 2019 Term Loan [Member] \| Interest-Only-Strip [Member]
Debt Instrument [Line Items]
Term loan, Interest payment period		18 months
Silicon Valley Bank [Member] \| 2019 Term Loan [Member] \| Principal-Only-Strip [Member]
Debt Instrument [Line Items]
Term loan, Interest payment period		36 months
Silicon Valley Bank [Member] \| 2019 Term Loan [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Repayments of lines of credit	15,000,000.0
Term loan, maximum borrowings	20,000,000.0
Term loan, remaining borrowings	$ 5,000,000.0			$ 5,000,000.0
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Term loan, principal amount			$ 201,300,000
Debt Instrument Maturity Date			May 01, 2024
Debt instrument interest rate			3.375%	3.375%
Debt instrument interest rate			3.375%	9.71%
Debt instrument frequency of periodic payment				semi-annually
Net proceeds from offering of convertible senior notes			$ 194,800,000
Debt discount and issuance costs			6,500,000	$ 68,900,000
Number of shares issued upon conversion of $1,000 debt principal amount \| shares				37.3413
Debt instrument convertible initial conversion price per share of common stock \| $ / shares				$ 26.78
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be convertible into common stock \| d				20
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible into common stock \| d				30
Percentage of the closing sales price of the entity's common stock that the conversion price must exceed in order for the notes to be convertible				130.00%
Number of consecutive business days immediately after any five consecutive trading day period during the note measurement period \| d				5
Number of consecutive trading days before five consecutive business days during the note measurement period \| d				10
Liability component of Convertible Notes recorded as long-term debt			136,700,000
Equity component of Convertible Notes recorded as additional paid-in capital			$ 64,500,000
Debt instrument effective interest rate period				7 years
Debt issuance costs				$ 6,500,000
Debt issuance costs allocated to liability component				4,400,000
Debt issuance costs allocated to equity component reduction to additional paid-in capital				2,100,000
Interest expense				4,000,000.0
Amortization of debt discount				$ 2,300,000
2024 Convertible Notes [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt instrument conversion obligation trading price as percentage of product common stock closing sale price and conversion rate				98.00%
2024 Convertible Notes [Member] \| Repaid In Year Two [Member]
Debt Instrument [Line Items]
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be convertible into common stock \| d				20
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible into common stock \| d				30
Percentage of the closing sales price of the entity's common stock that the conversion price must exceed in order for the notes to be convertible				130.00%
Debt instrument redemption date				May 06, 2020
Redemption price percentage of principal amount to be redeemed				100.00%

Flexion Therapeutics Inc 10-Q