Restructuring, Asset Impairment, and Transition Expenses - Reconciliation of Accrued Restructuring Costs (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Restructuring Reserve [Roll Forward]
Beginning Balance	$ 2.8	$ 2.3	$ 2.0	$ 0.9	$ 1.4	$ 1.0	$ 2.0	$ 1.0
Restructuring expense incurred	6.2	3.2	2.3	0.6	0.6	0.9	11.7	2.1
Cash payments	(3.0)	(2.7)	(2.0)	(0.9)	(1.1)	(0.5)
Ending Balance	6.0	2.8	2.3	0.6	0.9	1.4	6.0	0.6
2017 Altra Plan [Member]
Restructuring Reserve [Roll Forward]
Beginning Balance	1.3	1.5	2.0	0.4	0.7	0.2	2.0	0.2
Restructuring expense incurred	2.3	1.8	1.0	0.6	0.6	0.9	5.0
Cash payments	(1.3)	(2.0)	(1.5)	(0.7)	(0.9)	(0.4)
Ending Balance	2.3	1.3	1.5	0.3	0.4	0.7	2.3	0.3
2019 Altra Plan [Member]
Restructuring Reserve [Roll Forward]
Beginning Balance	1.5	0.8
Restructuring expense incurred	3.9	1.4	1.3				6.7
Cash payments	(1.7)	(0.7)	(0.5)
Ending Balance	$ 3.7	$ 1.5	$ 0.8				$ 3.7
2015 Altra Plan [Member]
Restructuring Reserve [Roll Forward]
Beginning Balance				0.5	0.7	0.8		0.8
Cash payments				(0.2)	(0.2)	(0.1)
Ending Balance				$ 0.3	$ 0.5	$ 0.7		$ 0.3