Income Taxes - Components of Federal and State Income Tax Provisions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income tax (expense) benefit:
Current expense (benefit)	$ (62.8)	$ 15.0	$ 72.4
Deferred expense (benefit)	(37.8)	24.6	(4.4)
Total income tax expense (benefit)	$ (100.6)	$ 39.6	$ 68.0