Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 20.5	$ 6.8	$ 10.8
State	6.9	6.0	8.5
Total current taxes	27.4	12.8	19.3
Deferred:
Federal	16.7	97.6	79.0
State	2.7	3.6	2.0
Total deferred taxes	19.4	101.2	81.0
Amortization of investment tax credits	(0.4)	(1.1)	(0.6)
Total income tax expense	$ 46.4	$ 112.9	$ 99.7