Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ (214.2)	$ 119.1	$ (2.4)
Reclassification Adjustment for Amounts Included in Net Income	(21.9)	(35.3)	0.2
Unrealized Holding Gains (Losses)	(236.1)	83.8	(2.2)
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0.0	1.7	(0.3)
Reclassification Adjustment for Amounts Included in Net Income	0.3	0.0	0.0
Foreign Currency Translation Adjustments	0.3	1.7	(0.3)
Unrecognized Postretirement Benefit Costs Arising During the Year	(8.0)	3.9	14.2
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	0.0	0.0	1.0
Amortization of Net Unrecognized Postretirement Benefit Costs	1.1	(0.6)	5.3
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax	1.1	(0.6)	6.3
Net Unrecognized Postretirement Benefit Costs	(6.9)	3.3	20.5
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	0.9	(8.0)	1.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0.3	1.3	0.0
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	1.2	(6.7)	1.6
Other Comprehensive Income (Loss) Before Income Taxes	(241.5)	82.1	19.6
Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Year Before Reclassification Adjustment	45.0	(38.2)	1.0
Reclassification Adjustment for Amounts Included in Net Income	4.6	12.3	(0.1)
Unrealized Holding Gains and Losses	49.6	(25.9)	0.9
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0.0	(0.6)	0.1
Reclassification Adjustment for Amounts Included in Net Income	(0.1)	0.0	0.0
Foreign Currency Translation Adjustment	(0.1)	(0.6)	0.1
Unrecognized Postretirement Benefit Costs Arising During the Year	1.7	(0.8)	(5.0)
Curtailment Cost Recognized	0.0	0.0	(0.4)
Amortization of Unrecognized Postretirement Benefit Costs	(0.2)	0.2	(1.9)
Total Reclassification Adjustments for Amounts Included in Net Income	(0.2)	0.2	(2.3)
Net Unrecognized Postretirement Benefit Costs	1.5	(0.6)	(7.3)
Gains and Losses on Cash Flow Hedges During the Year Before Reclassification Adjustment	(0.2)	2.8	(0.6)
Reclassification Adjustment for Amounts Included in Net Income	(0.1)	(0.4)	0.0
Gains and Losses on Cash Flow Hedges	(0.3)	2.4	(0.6)
Other Comprehensive Income Tax Benefit (Expense)	$ 50.7	$ (24.7)	$ (6.9)