Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ (152.1)	$ 96.9	$ (102.4)	$ 221.3
Reclassification Adjustment for Amounts Included in Net Income	(31.9)	0.7	(28.3)	(3.7)
Unrealized Holding Gains (Losses)	(184.0)	97.6	(130.7)	217.6
Foreign Currency Translation Adjustments	0.1	(0.2)	(0.8)	(0.2)
Amortization of Net Unrecognized Postretirement Benefit Costs	(6.1)	(1.9)	(11.5)	(3.9)
Other Comprehensive Income (Loss) Before Income Taxes	(177.8)	99.3	(120.0)	221.3
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Period Before Reclassification Adjustment	53.8	(34.1)	36.3	(78.0)
Reclassification Adjustment for Amounts Included in Net Income	11.2	(0.2)	9.9	1.3
Unrealized Holding Gains and Losses	65.0	(34.3)	46.2	(76.7)
Foreign Currency Translation Adjustments	0.0	0.1	0.3	0.1
Amortization of Net Unrecognized Postretirement Benefit Costs	(2.2)	(0.7)	(3.9)	(1.4)
Other Comprehensive Income Tax Benefit (Expense)	$ (62.8)	$ 34.9	$ (42.6)	$ 78.0