Income Taxes - Reconciliation of Change in Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Ending Balance	$ 17.0
Federal, State, and Foreign Tax [Member]
Income Tax Contingency [Line Items]
Beginning Balance	35.4	$ 46.1	$ 34.2
Additions for tax positions of prior years	1.7	1.8	14.1
Additions for tax positions	4.5	3.9	4.7
Reductions due to lapse of applicable statute of limitations	(4.1)	(16.4)	(6.9)
Ending Balance	37.5	35.4	46.1
Accrued Interest and Penalties [Member]
Income Tax Contingency [Line Items]
Beginning Balance	0.5	0.5	0.8
Additions for tax positions of prior years	0.3	0.2	0.2
Reductions due to lapse of applicable statute of limitations	(0.1)	(0.2)	(0.5)
Ending Balance	0.7	0.5	0.5
Gross Unrecognized Income Tax Benefits [Member]
Income Tax Contingency [Line Items]
Beginning Balance	35.9	46.6	35.0
Additions for tax positions of prior years	2.0	2.0	14.3
Additions for tax positions	4.5	3.9	4.7
Reductions due to lapse of applicable statute of limitations	(4.2)	(16.6)	(7.4)
Ending Balance	$ 38.2	$ 35.9	$ 46.6