Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ 96.9	$ (233.8)	$ 221.3	$ (257.7)
Reclassification Adjustment for Amounts Included in Net Income	0.7	(1.2)	(3.7)	(25.3)
Unrealized Holding Gains (Losses)	97.6	(235.0)	217.6	(283.0)
Foreign Currency Translation Adjustments	(0.2)	0.1	(0.2)	(0.1)
Amortization of Unrecognized Postretirement Benefit Costs	(1.9)	(6.7)	(3.9)	(12.1)
Other Comprehensive Income (Loss) Before Income Taxes	99.3	(228.2)	221.3	(271.0)
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Period Before Reclassification Adjustment	(34.1)	82.6	(78.0)	92.2
Reclassification Adjustment for Amounts Included in Net Income	(0.2)	0.4	1.3	8.8
Unrealized Holding Gains and Losses	(34.3)	83.0	(76.7)	101.0
Foreign Currency Translation Adjustment	0.1	(0.1)	0.1	0
Amortization of Unrecognized Postretirement Benefit Costs	(0.7)	(2.3)	(1.4)	(4.2)
Other Comprehensive Income Tax Benefit (Expense)	$ 34.9	$ (80.6)	$ 78.0	$ (96.8)