Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Comprehensive Income, Net of Tax, Including Portion Attributable to Noncontrolling Interest	$ 97.3	$ 156.3	$ 169.0	$ 346.5
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	138.0	176.4	192.0	343.1
Reclassification Adjustment for Amounts Included in Net Income	3.5	1.2	(27.2)	0.3
Unrealized Holding Gains (Losses)	141.5	177.6	164.8	343.4
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0	0.7	0.6	(1.4)
Reclassification Adjustment for Amounts Included in Net Income	0	0	0	2.1
Foreign Currency Translation Adjustments	0	0.7	0.6	0.7
Equity in Other Comprehensive Income (Loss) of Investee	0	7.9	0	2.9
Amortization of Unrecognized Postretirement Benefit Costs	2.1	0.2	6.5	0.8
Other Comprehensive Income (Loss) Before Income Taxes	143.6	186.4	171.9	347.8
Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Period Before Reclassification Adjustment	(49.2)	(62.2)	(68.4)	(121.3)
Reclassification Adjustment for Amounts Included in Net Income	(1.2)	(0.5)	9.6	(0.1)
Unrealized Holding Gains and Losses	(50.4)	(62.7)	(58.8)	(121.4)
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0	(0.3)	(0.2)	0.5
Reclassification Adjustment for Amounts Included in Net Income	0	0	0	(0.8)
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0	(0.3)	(0.2)	(0.3)
Equity in Other Comprehensive Income of Investee	0	(2.7)	0	(1.0)
Amortization of Unrecognized Postretirement Benefit Costs	(0.6)	(0.1)	(2.2)	(0.3)
Income Tax Benefit (Expense)	(51.0)	(65.8)	(61.2)	(123.0)
Other Comprehensive Income (Loss)	$ 92.6	$ 120.6	$ 110.7	$ 224.8