Other Comprehensive Income and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Other Comprehensive Income Before Income Taxes:
Unrealized Holding Gain (Loss) Arising During the Period, before Reclassification Adjustment and Tax	$ (233.8)	$ 76.7	$ (257.7)	$ 81.9
Reclassification Adjustment for Amounts Included in Net Income	(1.2)	(4.1)	(25.3)	(8.0)
Unrealized Holding Gains (Losses)	(235.0)	72.6	(283.0)	73.9
Foreign Currency Translation Adjustments	0.1	0.6	(0.1)	1.3
Amortization of Unrecognized Postretirement Benefit Costs	6.7	3.2	12.1	7.6
Other Comprehensive Income (Loss) Before Income Taxes	(228.2)	76.4	(271.0)	82.8
Income Tax Benefit (Expense):
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	82.6	(27.2)	92.2	(29.0)
Reclassification Adjustment for Amounts Included in Net Income	0.4	1.4	8.8	2.8
Unrealized Holding (Gains) Losses	83.0	(25.8)	101.0	(26.2)
Foreign Currency Translation Adjustment	(0.1)	(0.1)	0	(0.4)
Amortization of Unrecognized Postretirement Benefit Costs	(2.3)	(1.1)	(4.2)	(2.7)
Other Comprehensive Income Tax Benefit (Expense)	$ (80.6)	$ 27.0	$ (96.8)	$ 29.3