Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - January 1	$ 51.9	$ 69.5	$ 87.6
Additions for:
Current year tax positions	1.5	1.1	1.2
Prior year tax positions	0.0	0.0	0.2
Assumed in acquisition	0.0	0.0	0.2
Reductions for:
Prior year tax positions	(0.2)	(12.7)	(3.8)
Settlements	(5.5)	(0.2)	(10.9)
Lapse of statute	(1.9)	(5.8)	(5.0)
Unrecognized tax benefits - December 31	$ 45.8	$ 51.9	$ 69.5