Income Taxes - Comprehensive Income Tax Benefit and Expense (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
Operating Loss Carryforwards [Line Items]
Income Tax Benefit (Expense)	$ (1.1)	$ 11.8	$ (8.0)	$ (13.4)	$ (8.3)	$ (20.5)	$ (15.5)	$ 3.1	$ (10.7)	$ (41.2)	$ 9.2
Unrealized Depreciation (Appreciation) on Securities									49.6	(25.9)	0.9
Foreign Currency Translation Adjustments on Investments									(0.1)	(0.6)	0.1
Tax Effects from Postretirement Benefit Plans									1.5	(0.6)	(7.3)
Gain on Cash Flow Hedge									(0.3)	2.4	(0.6)
Tax Effects from Long-term Equity-based Compensation included in Paid-in Capital									0.0	0.0	(1.1)
Comprehensive Income Tax Benefit (Expense)									39.4	(66.4)	(1.0)
Discontinued Operations [Member]
Operating Loss Carryforwards [Line Items]
Income Tax Benefit (Expense)									(0.6)	(0.5)	(2.2)
Continuing Operations [Member]
Operating Loss Carryforwards [Line Items]
Income Tax Benefit (Expense)									$ (10.7)	$ (41.2)	$ 9.2