Income Taxes - Components of Provision for Income Tax Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal									$ 72.2	$ 16.1	$ 2.6
State and other									12.8	14.9	12.2
Total current expense									85.0	31.0	14.8
Deferred:
Federal									74.2	130.5	113.9
State and other									1.4	2.4	13.2
Total deferred expense									75.6	132.9	127.1
Total income tax expense related to continuing operations	$ 49.2	$ 43.1	$ 28.6	$ 39.7	$ 39.7	$ 42.1	$ 42.4	$ 39.7	$ 160.6	$ 163.9	$ 141.9