Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ (5.4)	$ 2.3	$ 215.9	$ (100.1)
Reclassification Adjustment for Amounts Included in Net Income	(2.9)	(2.3)	1.3	(30.6)
Unrealized Holding Gains (Losses)	(8.3)	0.0	217.2	(130.7)
Foreign Currency Translation Adjustments	0.0	(0.3)	0.0	(1.1)
Net Unrecognized Postretirement Benefit Costs Arising During the Period	0.0	0.0	(19.5)	0.0
Curtailment Cost Recognized	0.0	0.0	1.0	0.0
Amortization of Net Unrecognized Postretirement Benefit Costs	0.4	5.8	4.7	17.3
Total Reclassification Adjustments for Amounts Included in Net Income	0.4	5.8	5.7	17.3
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs	0.4	5.8	(13.8)	17.3
Other Comprehensive Income (Loss) Before Income Taxes	(7.9)	5.5	203.4	(114.5)
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Period Before Reclassification Adjustment	2.0	(0.7)	(76.0)	35.6
Reclassification Adjustment for Amounts Included in Net Income	1.1	0.9	(0.4)	10.8
Unrealized Holding Gains and Losses	3.1	0.2	(76.4)	46.4
Foreign Currency Translation Adjustments	0.0	0.1	0.0	0.4
Net Unrecognized Postretirement Benefit Costs Arising During the Period	0.0	0.0	6.9	0.0
Curtailment Loss Recognized	0.0	0.0	(0.4)	0.0
Amortization of Net Unrecognized Postretirement Benefit Costs	(0.2)	(2.1)	(1.7)	(6.0)
Total Reclassification Adjustments for Amounts Included in Net Income	(0.2)	(2.1)	(2.1)	(6.0)
Net Unrecognized Postretirement Benefit Costs	(0.2)	(2.1)	4.8	(6.0)
Other Comprehensive Income Tax Benefit (Expense)	$ (2.9)	$ 1.8	$ 71.6	$ (40.8)