Income Taxes - Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ 311.9	$ 351.2	$ 166.4
State	24.1	37.0	21.4
Total current	336.0	388.2	187.8
Deferred
Federal	8.3	(2.7)	(3.1)
State	2.2	7.3	0.4
Total deferred	10.5	4.6	(2.7)
Total income tax expense	$ 346.5	$ 392.8	$ 185.1