Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits – January 1	$ 46.2	$ 45.8	$ 51.9
Additions for:
Current year tax positions	0.9	1.5	1.5
Prior year tax positions	0.3	0.3	0.0
Reductions for:
Prior year tax positions	0.0	(0.1)	(0.2)
Expiration of statute of limitations	(4.7)	(1.2)	(1.9)
Settlements	(1.2)	(0.1)	(5.5)
Unrecognized tax benefits – December 31	$ 41.5	$ 46.2	$ 45.8