Income Taxes - Statutory Federal Income Tax Expense, Effective Income Tax Expense, Rates (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
Income Tax Disclosure [Abstract]
Statutory Federal Income Tax Expense									$ (56.4)	$ (1.2)	$ (35.1)
Statutory Federal Income Tax Expense, Rate, percentage									35.00%	35.00%	35.00%
Tax-exempt Income and Dividends Received Deduction									$ 9.8	$ 9.8	$ 9.8
Tax-exempt Income and Dividends Received Deduction, Rate, percentage									(6.00%)	(279.50%)	(9.70%)
Effective Income Tax Rate Reconciliation, Unrecognized Tax Benefit (Expense), Amount									$ 0.0	$ 0.0	$ 2.1
Effective Income Tax Rate Reconciliation, Unrecognized Tax Benefit (Expense), Percent									(0.00%)	(0.00%)	(2.10%)
Effective Income Tax Rate Reconciliation, Indemnification Recoveries, Amount									$ 0.0	$ 0.2	$ 3.7
Effective Income Tax Rate Reconciliation, Indemnification Recoveries, Percent									(0.00%)	(6.50%)	(3.60%)
State Income Taxes									$ 0.1	$ (0.6)	$ (0.6)
State Income Taxes, Percentage									(0.10%)	16.90%	0.50%
Nondeductible Expense, Share-based Compensation Cost, Amount									$ 0.4	$ 0.0	$ 0.0
Nondeductible Expense, Share-based Compensation Cost, Percent									(0.20%)	0.00%	0.00%
Change in Enacted Tax Rate, Amount									$ (7.4)	$ 0.0	$ 0.0
Change in Enacted Tax Rate, Percent									(4.60%)	0.00%	0.00%
Other, Net									$ (2.5)	$ 1.0	$ 0.0
Other, Net, Percent									1.50%	(28.90%)	0.00%
Income Tax Expense	$ (8.3)	$ (20.5)	$ (15.5)	$ 3.1	$ (11.5)	$ 14.9	$ 1.5	$ 4.3	$ (41.2)	$ 9.2	$ (20.1)
Effective Income Tax Expense, Rate, percentage									25.60%	(263.00%)	20.10%