Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ (2.4)	$ (151.9)	$ 259.7
Reclassification Adjustment for Amounts Included in Net Income	0.2	(25.4)	(25.1)
Unrealized Holding Gains (Losses)	(2.2)	(177.3)	234.6
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	(0.3)	(1.4)	(1.0)
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.0	0.0
Foreign Currency Translation Adjustments	(0.3)	(1.4)	(1.0)
Unrecognized Postretirement Benefit Costs Arising During the Year	14.2	3.0	(106.4)
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, before Tax	1.0	0.0	0.0
Amortization of Net Unrecognized Postretirement Benefit Costs	5.3	23.1	7.9
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax	6.3	23.1	7.9
Net Unrecognized Postretirement Benefit Costs	20.5	26.1	(98.5)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	1.6	0.0	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0.0	0.0	0.0
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	1.6	0.0	0.0
Other Comprehensive Income (Loss) Before Income Taxes	19.6	(152.6)	135.1
Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Year Before Reclassification Adjustment	1.0	53.9	(91.6)
Reclassification Adjustment for Amounts Included in Net Income	(0.1)	8.9	8.8
Unrealized Holding Gains and Losses	0.9	62.8	(82.8)
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0.1	0.5	0.4
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.0	0.0
Foreign Currency Translation Adjustment	0.1	0.5	0.4
Unrecognized Postretirement Benefit Costs Arising During the Year	(5.0)	(1.1)	37.5
Curtailment Cost Recognized	(0.4)	0.0	0.0
Amortization of Unrecognized Postretirement Benefit Costs	(1.9)	(8.0)	(2.8)
Total Reclassification Adjustments for Amounts Included in Net Income	(2.3)	(8.0)	(2.8)
Net Unrecognized Postretirement Benefit Costs	(7.3)	(9.1)	34.7
Gain on Cash Flow Hedge During the Year Before Reclassification Adjustment	(0.6)	0.0	0.0
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.0	0.0
Gain on Cash Flow Hedge	(0.6)	0.0	0.0
Other Comprehensive Income Tax Benefit (Expense)	$ (6.9)	$ 54.2	$ (47.7)