Restructuring, Asset Impairment, and Transition Expenses - Reconciliation of Accrued Restructuring Costs (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Restructuring Reserve [Roll Forward]
Beginning Balance	$ 3.9	$ 4.1	$ 2.3	$ 2.0	$ 4.1	$ 2.0
Restructuring expense incurred	1.5	1.6	3.2	2.3	3.1	5.5
Cash payments	(1.7)	(1.8)	(2.7)	(2.0)
Ending Balance	3.7	3.9	2.8	2.3	3.7	2.8
2017 Altra Plan [Member]
Restructuring Reserve [Roll Forward]
Beginning Balance	1.3	1.5	1.5	2.0	1.5	2.0
Restructuring expense incurred	0.3	0.2	1.8	1.0	0.5	2.8
Cash payments	(0.6)	(0.4)	(2.0)	(1.5)
Ending Balance	1.0	1.3	1.3	1.5	1.0	1.3
2019 Altra Plan [Member]
Restructuring Reserve [Roll Forward]
Beginning Balance	2.6	2.6	0.8		2.6
Restructuring expense incurred	1.2	1.4	1.4	1.3	2.6	2.7
Cash payments	(1.1)	(1.4)	(0.7)	(0.5)
Ending Balance	$ 2.7	$ 2.6	$ 1.5	$ 0.8	$ 2.7	$ 1.5