Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ 433.4	$ (214.2)	$ 119.1
Reclassification Adjustment for Amounts Included in Net Income	(28.1)	(21.9)	(35.3)
Unrealized Holding Gains (Losses)	405.3	(236.1)	83.8
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0.0	0.0	1.7
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.3	0.0
Foreign Currency Translation Adjustments	0.0	0.3	1.7
Unrecognized Postretirement Benefit Costs Arising During the Year	(4.8)	(8.0)	3.9
Amortization of Net Unrecognized Postretirement Benefit Costs	(3.0)	1.1	(0.6)
Total Reclassification Adjustments for Amounts Included in Net Income	(3.0)	1.1	(0.6)
Net Unrecognized Postretirement Benefit Costs	(7.8)	(6.9)	3.3
Gains (Losses) on Cash Flow Hedges During the Year Before Reclassification Adjustment	0.0	0.9	(8.0)
Reclassification Adjustment for Amounts Included in Net Income	0.4	0.3	1.3
Gains (Losses) on Cash Flow Hedges	0.4	1.2	(6.7)
Other Comprehensive Income (Loss) Before Income Taxes	397.9	(241.5)	82.1
Income Tax Benefit (Expense):
Unrealized Holding Gains (Losses) Arising During the Year Before Reclassification Adjustment	(91.0)	45.0	(38.2)
Reclassification Adjustment for Amounts Included in Net Income	5.8	4.6	12.3
Unrealized Holding Gains and Losses	(85.2)	49.6	(25.9)
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0.0	0.0	(0.6)
Reclassification Adjustment for Amounts Included in Net Income	0.0	(0.1)	0.0
Foreign Currency Translation Adjustment	0.0	(0.1)	(0.6)
Unrecognized Postretirement Benefit Costs Arising During the Year	1.0	1.7	(0.8)
Amortization of Unrecognized Postretirement Benefit Costs	0.7	(0.2)	0.2
Total Reclassification Adjustments for Amounts Included in Net Income	0.7	(0.2)	0.2
Net Unrecognized Postretirement Benefit Costs	1.7	1.5	(0.6)
Gains (Losses) on Cash Flow Hedges During the Year Before Reclassification Adjustment	0.0	(0.2)	2.8
Reclassification Adjustment for Amounts Included in Net Income	(0.1)	(0.1)	(0.4)
Gains (Losses) on Cash Flow Hedges	(0.1)	(0.3)	2.4
Other Comprehensive Income Tax Benefit (Expense)	$ (83.6)	$ 50.7	$ (24.7)