Income Taxes - Comprehensive Income Tax Benefit and Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Loss Carryforwards [Line Items]
Income Tax Expense	$ 11.5	$ (14.9)	$ (1.5)	$ (4.3)	$ (2.9)	$ 11.8	$ 6.9	$ 4.3	$ (9.2)	$ 20.1	$ 47.6
Unrealized Depreciation (Appreciation) on Securities									0.9	62.8	(82.8)
Foreign Currency Translation Adjustments on Investments									0.1	0.5	0.4
Tax Effects from Postretirement Benefit Plans									(7.3)	(9.1)	34.7
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax									(0.6)	0.0	0.0
Tax Effects from Long-term Equity-based Compensation included in Paid-in Capital									(1.1)	(1.0)	(1.0)
Comprehensive Income Tax Benefit (Expense)									1.0	(30.0)	97.4
Discontinued Operations [Member]
Operating Loss Carryforwards [Line Items]
Income Tax Expense									2.2	3.1	1.1
Continuing Operations [Member]
Operating Loss Carryforwards [Line Items]
Income Tax Expense									$ (9.2)	$ 20.1	$ 47.6