Income Taxes - Income Tax Provision (Benefit) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Current tax provision (benefit)	$ (2.9)	$ 10.8	$ 12.6	$ 18.8
Future tax deductions	(2.5)	(3.0)	(6.1)	1.9
Valuation allowance change	0.8	(0.8)	(0.3)	0.4
Income tax provision (benefit)	$ (4.6)	$ 7.0	$ 6.2	$ 21.1