Income Taxes (Reconciliation of Income Tax Rate) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Provision at the U.S. statutory rate of 35%	$ 141.0	$ 98.9	$ 111.2
U.S. statutory rate (as a percent)	35.00%	35.00%	35.00%
Increase (reduction) in tax expense resulting from:
State income tax, net of federal income tax benefit	$ 12.8	$ 8.0	$ 8.8
Other permanent items	4.2	(9.1)	(8.2)
Tax credits, net of unrecognized tax benefits	(27.9)	(0.7)	0.0
Change in unrecognized tax benefits	(0.3)	(0.9)	0.2
Change in valuation allowance	(4.3)	(0.6)	0.2
Foreign taxes at rates other than 35% and miscellaneous other	(1.4)	(0.2)	(2.7)
Total provision for income taxes	$ 124.1	$ 95.4	$ 109.5