Description of Business and Summary of Significant Accounting Policies - Reconciliation of Basic to Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net income attributable to Altra Industrial Motion Corp.	$ 1,668	[1]	$ 5,313	[1]	$ 9,349	[1]	$ 8,810	[1]	$ 6,108	[1]	$ 10,221	[1]	$ 9,679	[1]	$ 9,398	[1]	$ 25,140	$ 35,406	$ 40,167
Shares used in net income per common share - basic																	25,719	26,064	26,713
Dilutive effect of the equity premium on Convertible Notes at the average price of common stock																	132	43	612
Incremental shares of unvested restricted common stock																	21	2	78
Shares used in net income per common share - diluted																	25,872	26,109	27,403
Earnings per share:
Basic net income attributable to Altra Industrial Motion Corp.	$ 0.06		$ 0.21		$ 0.36		$ 0.34		$ 0.23		$ 0.39		$ 0.37		$ 0.36		$ 0.97	$ 1.36	$ 1.50
Diluted net income attributable to Altra Industrial Motion Corp.	$ 0.06		$ 0.21		$ 0.36		$ 0.34		$ 0.23		$ 0.39		$ 0.37		$ 0.36		$ 0.97	$ 1.36	$ 1.47

[1] Includes restructuring costs by quarter