Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ (33.8)	$ 98.3	$ (291.5)	$ 180.2
Reclassification Adjustment for Amounts Included in Net Income	0.4	(50.6)	(24.9)	(58.6)
Unrealized Holding Gains (Losses)	(33.4)	47.7	(316.4)	121.6
Foreign Currency Translation Adjustments	0.2	0.2	0.1	1.5
Amortization of Unrecognized Postretirement Benefit Costs	6.3	4.4	18.4	12.0
Other Comprehensive Income (Loss) Before Income Taxes	(26.9)	52.3	(297.9)	135.1
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding (Gains) Losses Arising During the Period Before Reclassification Adjustment	11.9	(34.8)	104.1	(63.8)
Reclassification Adjustment for Amounts Included in Net Income	(0.1)	17.8	8.7	20.6
Unrealized Holding (Gains) Losses	11.8	(17.0)	112.8	(43.2)
Foreign Currency Translation Adjustment	0	(0.1)	0	(0.5)
Amortization of Unrecognized Postretirement Benefit Costs	(2.3)	(1.6)	(6.5)	(4.3)
Other Comprehensive Income Tax Benefit (Expense)	$ (9.5)	$ 18.7	$ (106.3)	$ 48.0