Description of Business and Summary of Significant Accounting Policies - Reconciliation of Basic to Diluted Shares Outstanding (Detail) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[1]	Sep. 30, 2016	[1]	Jun. 30, 2016	[1]	Mar. 31, 2016	[1]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net income attributable to Altra Industrial Motion Corp.	$ 12,440		$ 13,277		$ 15,384		$ 10,326		$ 1,668		$ 5,313		$ 9,349		$ 8,810		$ 51,427	$ 25,140	$ 35,406
Shares used in net income per common share – basic																	28,949	25,719	26,064
Dilutive effect of the equity premium on Convertible Notes at the average price of common stock																		132	43
Incremental shares of unvested restricted common stock																	115	21	2
Shares used in net income per common share – diluted																	29,064	25,872	26,109

[1] Includes restructuring costs by quarter