Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current tax expense (benefit):
Federal	$ 24.0	$ 54.1	$ 85.7
Total current tax expense (benefit)	24.0	54.1	85.7
Deferred tax expense (benefit):
Federal	(2.6)	(1.3)	(11.2)
Total deferred tax expense (benefit)	(2.6)	(1.3)	(11.2)
Income tax expense (benefit)	$ 21.4	$ 52.8	$ 74.5