Income Taxes - Schedule of Income Taxes Calculated on Income before Income Taxes and Provision for Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income before provision for income taxes	$ 437.9	$ 441.8	$ 416.2
Federal statutory income tax rates	35.00%	35.00%	35.00%
Income taxes, at statutory income tax rate	$ 153.3	$ 154.6	$ 145.7
State income tax, net of federal income tax	14.8	14.8	14.5
AFUDC-equity	(8.4)	(6.0)	(3.7)
Tax credits	(6.8)	0.0	0.0
Other	(0.7)	2.1	(0.6)
Total income tax expense	$ 152.2	$ 165.5	$ 155.9
Income tax expense as a percent of income from continuing operations, before income taxes	34.80%	37.50%	37.50%