Income Taxes - Comprehensive Income Tax Benefit and Expense (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
Operating Loss Carryforwards [Line Items]
Income Tax Benefit (Expense)	$ (8.3)	$ (20.5)	$ (15.5)	$ 3.1	$ (11.5)	$ 14.9	$ 1.5	$ 4.3	$ (41.2)	$ 9.2	$ (20.1)
Unrealized Depreciation (Appreciation) on Securities									(25.9)	0.9	62.8
Foreign Currency Translation Adjustments on Investments									(0.6)	0.1	0.5
Tax Effects from Postretirement Benefit Plans									(0.6)	(7.3)	(9.1)
Gain on Cash Flow Hedge									2.4	(0.6)	0.0
Tax Effects from Long-term Equity-based Compensation included in Paid-in Capital									0.0	(1.1)	(1.0)
Comprehensive Income Tax Benefit (Expense)									(66.4)	(1.0)	30.0
Discontinued Operations [Member]
Operating Loss Carryforwards [Line Items]
Income Tax Benefit (Expense)									(0.5)	(2.2)	(3.1)
Continuing Operations [Member]
Operating Loss Carryforwards [Line Items]
Income Tax Benefit (Expense)									$ (41.2)	$ 9.2	$ (20.1)