Income Taxes - Unrecognized Tax Benefits and Effect on Effective Income Tax Rate (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]
Beginning balance of unrecognized tax benefits	$ 162.6	$ 131.3	$ 18.9
Additions for tax positions of current year	7.3	11.1	1.2
Additions for tax positions of prior years	49.3	23.4	111.8
Reductions for tax positions of prior years	(4.3)	(1.4)	(0.6)
Reductions for lapses of statutes of limitation	(0.6)	(1.8)	0.0
Ending balance of unrecognized tax benefits	$ 214.3	$ 162.6	$ 131.3