Unaudited Quarterly Results of Operations - Schedule of Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net Sales	$ 223,322		$ 214,623		$ 223,357		$ 215,435		$ 172,647		$ 173,132		$ 182,674		$ 180,453		$ 876,737	$ 708,906	$ 746,652
Gross Profit	68,470		69,013		72,126		66,167		55,127		54,175		58,200		54,630		275,776	222,132	228,463
Net income attributable to Altra Industrial Motion Corp.	$ 12,440	[1]	$ 13,277	[1]	$ 15,384	[1]	$ 10,326	[1]	$ 1,668	[1]	$ 5,313	[1]	$ 9,349	[1]	$ 8,810	[1]	$ 51,427	$ 25,140	$ 35,406
Basic net income attributable to Altra Industrial Motion Corp.	$ 0.43		$ 0.46		$ 0.53		$ 0.36		$ 0.06		$ 0.21		$ 0.36		$ 0.34		$ 1.78	$ 0.97	$ 1.36
Diluted net income attributable to Altra Industrial Motion Corp.	$ 0.43		$ 0.46		$ 0.53		$ 0.36		$ 0.06		$ 0.21		$ 0.36		$ 0.34		$ 1.77	$ 0.97	$ 1.36
Restructuring Costs	$ 367		$ 680		$ 1,198		$ 1,898		$ 3,258		$ 3,397		$ 1,641		$ 1,553

[1] Includes restructuring costs by quarter