Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Current income taxes, Federal	$ 52.7	$ 38.2	$ 54.8
Current income taxes, State	11.8	8.4	8.9
Deferred income taxes, Federal	75.7	102.9	79.0
Deferred income taxes, State	11.0	14.5	13.5
Investment tax credits, net of amortization	1.0	1.5	(0.3)
Total income tax expense	$ 152.2	$ 165.5	$ 155.9