Other Comprehensive Income and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains Arising During the Period Before Reclassification Adjustment	$ 57.7	$ 123.1	$ 91.3	$ 221.3
Reclassification Adjustment for Amounts Included in Net Income	(19.0)	1.7	(24.1)	4.2
Unrealized Holding Gains	38.7	124.8	67.2	225.5
Foreign Currency Translation Adjustments	0.7	(0.1)	0.8	0.0
Net Unrecognized Postretirement Benefit Costs Arising During the Period	0.0	(18.7)	0.0	(19.5)
Curtailment Cost Recognized	0.0	1.0	0.0	1.0
Amortization of Net Unrecognized Postretirement Benefit Costs	(0.2)	1.7	(0.3)	4.3
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax	(0.2)	2.7	(0.3)	5.3
Net Unrecognized Postretirement Benefit Costs	0.2	16.0	0.3	14.2
Loss on Cash Flow Hedges During the Period Before Reclassification Adjustment	(5.6)	0.0	(7.6)	0.0
Reclassification Adjustment for Amounts Included in Net Income	(1.1)	0.0	(1.1)	0.0
Loss on Cash Flow Hedges	(4.5)	0.0	(6.5)	0.0
Other Comprehensive Income Before Income Taxes	34.7	108.7	61.2	211.3
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Period Before Reclassification Adjustment	(20.2)	(43.4)	(32.1)	(78.0)
Reclassification Adjustment for Amounts Included in Net Income	6.6	(0.6)	8.4	(1.5)
Unrealized Holding Gains	(13.6)	(44.0)	(23.7)	(79.5)
Foreign Currency Translation Adjustments	(0.3)	0.0	(0.3)	0.0
Net Unrecognized Postretirement Benefit Costs Arising During the Period	0.0	(6.6)	0.0	6.9
Curtailment Loss Recognized	0.0	(0.4)	0.0	(0.4)
Amortization of Net Unrecognized Postretirement Benefit Costs	0.1	(0.5)	0.1	(1.5)
Total Reclassification Adjustments for Amounts Included in Net Income	0.1	(0.9)	0.1	(1.9)
Net Unrecognized Postretirement Benefit Costs	0.1	5.7	0.1	5.0
Loss on Cash Flow Hedges During the Period Before Reclassification Adjustment	2.0	0.0	2.7	0.0
Reclassification Adjustment for Amounts Included in Net Income	(0.4)	0.0	(0.4)	0.0
Loss on Cash Flow Hedges	1.6	0.0	2.3	0.0
Other Comprehensive Income Tax Expense	$ 12.2	$ 38.3	$ 21.6	$ 74.5