Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains Arising During the Period Before Reclassification Adjustment	$ 19.2	$ (5.4)	$ 110.5	$ 215.9
Reclassification Adjustment for Amounts Included in Net Income	(3.6)	(2.9)	(27.7)	1.3
Unrealized Holding Gains	15.6	(8.3)	82.8	217.2
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0.1	0.0	0.9	0.0
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.0	0.0	0.0
Foreign Currency Translation Adjustments	0.1	0.0	0.9	0.0
Net Unrecognized Postretirement Benefit Costs Arising During the Period	0.0	0.0	0.0	(19.5)
Curtailment Cost Recognized	0.0	0.0	0.0	1.0
Amortization of Net Unrecognized Postretirement Benefit Costs	(0.1)	0.4	(0.4)	4.7
Total Reclassification Adjustments for Amounts Included in Net Income	(0.1)	0.4	(0.4)	5.7
Net Unrecognized Postretirement Benefit Costs	(0.1)	0.4	(0.4)	(13.8)
Loss on Cash Flow Hedges During the Period Before Reclassification Adjustment	(0.3)	0.0	(7.9)	0.0
Reclassification Adjustment for Amounts Included in Net Income	0.2	0.0	1.3	0.0
Loss on Cash Flow Hedges	(0.1)	0.0	(6.6)	0.0
Other Comprehensive Income (Loss) Before Income Taxes	15.5	(7.9)	76.7	203.4
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Period Before Reclassification Adjustment	(7.0)	2.0	(39.1)	(76.0)
Reclassification Adjustment for Amounts Included in Net Income	1.3	1.1	9.7	(0.4)
Unrealized Holding Gains	(5.7)	3.1	(29.4)	(76.4)
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0.1	0.0	0.4	0.0
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.0	0.0	0.0
Foreign Currency Translation Adjustments	(0.1)	0.0	(0.4)	0.0
Net Unrecognized Postretirement Benefit Costs Arising During the Period	0.0	0.0	0.0	6.9
Curtailment Loss Recognized	0.0	0.0	0.0	(0.4)
Amortization of Net Unrecognized Postretirement Benefit Costs	0.1	(0.2)	0.2	(1.7)
Total Reclassification Adjustments for Amounts Included in Net Income	0.1	(0.2)	0.2	(2.1)
Net Unrecognized Postretirement Benefit Costs	0.1	(0.2)	0.2	4.8
Loss on Cash Flow Hedges During the Period Before Reclassification Adjustment	0.0	0.0	2.7	0.0
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.0	(0.4)	0.0
Loss on Cash Flow Hedges	0.0	0.0	2.3	0.0
Other Comprehensive Income Tax Benefit (Expense)	$ 5.7	$ (2.9)	$ 27.3	$ 71.6