Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024	Jul. 24, 2023	Apr. 30, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options shares outstanding (in shares)		1,964,720		1,964,720		1,650,390
Common stock, shares issued (in shares)		7,323,106		7,323,106		7,303,629
Compensation expense related to the unvested option		$ 300,000		$ 300,000
Number of options outstanding, Options exercised (in shares)				0
Payments of dividends of common stock				$ 0
Expected dividend yield (as a percent)		0.00%	0.00%	0.00%	0.00%
Share-Based Payment Arrangement, Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs vesting tranches (as a percent)				33.00%
Share-Based Payment Arrangement, Tranche Two
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs vesting tranches (as a percent)				33.00%
Share-Based Payment Arrangement, Tranche Three
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs vesting tranches (as a percent)				33.00%
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares, granted (in shares)				0
Performance-Based Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 3,000		$ 300,000
Share-Based Payment Arrangement, Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation cost		$ 8,000,000.0		$ 8,000,000.0
Estimated weighted average amortization period (in years)				2 years 4 months 24 days
Employee Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Incremental cost				$ 400,000	$ 600,000
Look back period (in months)				6 months
2022 Stock Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted (in shares)								300,000
Stock options shares outstanding (in shares)		98,300		98,300
Shares available for future issuance (in shares)		201,700		201,700
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options shares outstanding (in shares)		1,743,108		1,743,108
Shares available for future issuance (in shares)		184,647		184,647
Weighted average grant date fair value of options granted (in dollars per share)		$ 3.21		$ 4.73
2018 Equity Incentive Plan \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum number of shares authorized available for issuance (in shares)	160,069
Increase in number of shares reserved for issuance as percentage of total number of shares of capital stock outstanding on last date of preceding calendar year (as a percent)				5.00%
2018 Equity Incentive Plan \| Maximum \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum number of shares authorized available for issuance (in shares)	400,000
2018 Equity Incentive Plan \| Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options shares outstanding (in shares)		1,127		1,127
Option grant vesting period (in years)				3 years
2018 Equity Incentive Plan \| Incentive Stock Options \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum number of shares issued upon exercise of incentive stock options (in shares)		1,250,000		1,250,000
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option grant vesting period (in years)				4 years
Options to purchase of common stock outstanding (in shares)		123,312		123,312
2015 Equity Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted expiry period (in years)				10 years
2018 Employee Stock Purchase Plan \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future issuance (in shares)	20,000	48,214		48,214
Additional shares available for future issuance (in shares)						0
Common stock, shares issued (in shares)		84,996		84,996
2018 Employee Stock Purchase Plan \| Maximum \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of annual increase in number of common stock shares (as a percent)				1.00%
Annual increase in number of common stock shares (in shares)	37,500
Option price per share of common stock at fair market value (as a percent)				85.00%
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Modified exercise price (in dollars per share)							$ 22.80

Kezar Life Sciences, Inc. 10-Q