Income Taxes - Statutory Federal Income Tax Expense, Effective Income Tax Expense, Rates (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Statutory Federal Income Tax Expense									$ (41.8)	$ (56.4)	$ (1.2)
Statutory Federal Income Tax Expense, Rate, percentage									21.00%	35.00%	35.00%
Tax-exempt Income and Dividends Received Deduction									$ 4.8	$ 9.8	$ 9.8
Tax-exempt Income and Dividends Received Deduction, Rate, percentage									(2.40%)	(6.00%)	(279.50%)
Effective Income Tax Rate Reconciliation, Indemnification Recoveries, Amount									$ 0.0	$ 0.0	$ 0.2
Effective Income Tax Rate Reconciliation, Indemnification Recoveries, Percent									(0.00%)	(0.00%)	(6.50%)
State Income Taxes									$ (0.1)	$ 0.1	$ (0.6)
State Income Taxes, Percentage									0.10%	(0.10%)	16.90%
Change in Enacted Tax Rate, Amount									$ (26.4)	$ (7.4)	$ 0.0
Change in Enacted Tax Rate, Percent									(13.30%)	(4.60%)	0.00%
Other, Net									$ 0.0	$ (2.1)	$ 1.0
Other, Net, Percent									0.00%	1.30%	(28.90%)
Income Tax Expense	$ (1.1)	$ 11.8	$ (8.0)	$ (13.4)	$ (8.3)	$ (20.5)	$ (15.5)	$ 3.1	$ (10.7)	$ (41.2)	$ 9.2
Effective Income Tax Expense, Rate, percentage									5.40%	25.60%	(263.00%)