Net Income per Share - Reconciliation of Basic to Diluted Net Income per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Net income attributable to Altra Industrial Motion Corp.	$ 9,349	$ 9,679	$ 18,159	$ 19,078
Shares used in net income per common share - basic (in shares)	25,699	26,280	25,699	26,204
Dilutive effect of the equity premium on Convertible Notes at the average price of common stock (in shares)	232	127	9	37
Incremental shares of unvested restricted common stock (in shares)	37	43	85	46
Shares used in net income per common share - diluted (in shares)	25,968	26,450	25,793	26,287
Earnings per share:
Basic net income attributable to Altra Industrial Motion Corp. (in usd per share)	$ 0.36	$ 0.37	$ 0.71	$ 0.73
Diluted net income attributable to Altra Industrial Motion Corp. (in usd per share)	$ 0.36	$ 0.37	$ 0.70	$ 0.73