Consolidated statements of comprehensive (loss) income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (99,407)	$ 10,163	$ 10,931
Other comprehensive income (loss), net of tax:
Foreign currency translation (net of tax effect of $0, $(60) and $953, respectively)	2,351	20	(2,115)
Retirement related benefit plans (net of tax effect of $0, $195 and $(36), respectively)	(159)	(576)	(253)
Other comprehensive income (loss)	2,192	(556)	(2,368)
Comprehensive (loss) income	(97,215)	9,607	8,563
Less: comprehensive loss attributable to noncontrolling interest			(1)
Comprehensive (loss) income attributable to Altair Engineering Inc.	$ (97,215)	$ 9,607	$ 8,564