Other Comprehensive Income and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Other Comprehensive Income Before Income Taxes:
Unrealized Holding Gain (Loss) Arising During the Period, before Reclassification Adjustment and Tax	$ 98.3	$ 138.0	$ 180.2	$ 192.0
Reclassification Adjustment for Amounts Included in Net Income	(50.6)	3.5	(58.6)	(27.2)
Unrealized Holding Gains (Losses)	47.7	141.5	121.6	164.8
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0.2	0	1.5	0.6
Reclassification Adjustment for Amounts Included in Net Income	0	0	0	0
Foreign Currency Translation Adjustments	0.2	0	1.5	0.6
Amortization of Unrecognized Postretirement Benefit Costs	4.4	2.1	12.0	6.5
Other Comprehensive Income Before Income Taxes	52.3	143.6	135.1	171.9
Income Tax Benefit (Expense):
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	(34.8)	(49.2)	(63.8)	(68.4)
Reclassification Adjustment for Amounts Included in Net Income	17.8	(1.2)	20.6	9.6
Unrealized Holding (Gains) Losses	(17.0)	(50.4)	(43.2)	(58.8)
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	(0.1)	0	(0.5)	(0.2)
Reclassification Adjustment for Amounts Included in Net Income	0	0	0	0
Foreign Currency Translation Adjustment	(0.1)	0	(0.5)	(0.2)
Amortization of Unrecognized Postretirement Benefit Costs	(1.6)	(0.6)	(4.3)	(2.2)
Other Comprehensive Income Tax Expense	$ 18.7	$ 51.0	$ 48.0	$ 61.2