GOODWILL AND OTHER INTANGIBLE ASSETS - Amortization of the Acquired Intangible Assets Other than Goodwill (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Amortized intangible assets
Gross Carrying Amount	$ 154,239	$ 154,776
Accumulated Amortization	(63,443)	(59,655)
Customer Relationships
Amortized intangible assets
Gross Carrying Amount	143,184	143,479
Accumulated Amortization	$ (53,970)	(50,076)
Customer Relationships \| Minimum
Finite-Lived Intangible Assets [Line Items]
Amortization Period	10 years
Customer Relationships \| Maximum
Finite-Lived Intangible Assets [Line Items]
Amortization Period	14 years
Patents
Finite-Lived Intangible Assets [Line Items]
Amortization Period	28 years
Amortized intangible assets
Gross Carrying Amount	$ 650	650
Accumulated Amortization	$ (619)	(616)
Technology
Finite-Lived Intangible Assets [Line Items]
Amortization Period	15 years
Amortized intangible assets
Gross Carrying Amount	$ 2,500	2,500
Accumulated Amortization	(1,185)	(1,144)
Trade Names
Amortized intangible assets
Gross Carrying Amount	7,446	7,495
Accumulated Amortization	$ (7,210)	(7,167)
Trade Names \| Minimum
Finite-Lived Intangible Assets [Line Items]
Amortization Period	3 years
Trade Names \| Maximum
Finite-Lived Intangible Assets [Line Items]
Amortization Period	5 years
License Agreements
Finite-Lived Intangible Assets [Line Items]
Amortization Period	10 years
Amortized intangible assets
Gross Carrying Amount	$ 0	185
Accumulated Amortization	0	(185)
Other
Amortized intangible assets
Gross Carrying Amount	459	467
Accumulated Amortization	$ (459)	$ (467)
Other \| Minimum
Finite-Lived Intangible Assets [Line Items]
Amortization Period	7 years
Other \| Maximum
Finite-Lived Intangible Assets [Line Items]
Amortization Period	15 years

LYDALL INC /DE/ 10-Q