Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ (151.9)	$ 259.7	$ (324.9)
Reclassification Adjustment for Amounts Included in Net Income	(25.4)	(25.1)	(37.9)
Unrealized Holding Gains (Losses)	(177.3)	234.6	(362.8)
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	(1.4)	(1.0)	0.2
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.0	0.0
Foreign Currency Translation Adjustments	(1.4)	(1.0)	0.2
Unrecognized Postretirement Benefit Costs Arising During the Year	3.0	(106.4)	61.6
Amortization of Net Unrecognized Postretirement Benefit Costs	23.1	7.9	25.0
Net Unrecognized Postretirement Benefit Costs	26.1	(98.5)	86.6
Other Comprehensive Income (Loss) Before Income Taxes	(152.6)	135.1	(276.0)
Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Year Before Reclassification Adjustment	53.9	(91.6)	115.9
Reclassification Adjustment for Amounts Included in Net Income	8.9	8.8	13.2
Unrealized Holding Gains and Losses	62.8	(82.8)	129.1
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0.5	0.4	(0.1)
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.0	0.0
Foreign Currency Translation Adjustment	0.5	0.4	(0.1)
Unrecognized Postretirement Benefit Costs Arising During the Year	(1.1)	37.5	(21.6)
Amortization of Unrecognized Postretirement Benefit Costs	(8.0)	(2.8)	(8.8)
Net Unrecognized Postretirement Benefit Costs	(9.1)	34.7	(30.4)
Other Comprehensive Income Tax Benefit (Expense)	$ 54.2	$ (47.7)	$ 98.6