Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - January 1	$ 69.5	$ 87.6	$ 160.1
Additions for:
Current year tax positions	1.1	1.2	3.4
Prior year tax positions	0.0	0.2	1.8
Assumed in acquisition	0.0	0.2	1.9
Reductions for:
Prior year tax positions	(12.7)	(3.8)	(56.6)
Settlements	(0.2)	(10.9)	(19.0)
Lapse of statute	(5.8)	(5.0)	(4.0)
Unrecognized tax benefits - December 31	$ 51.9	$ 69.5	$ 87.6