Schedule 2 - Parent Company Financial Statements - Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Statement of Income Captions [Line Items]
Net Income	$ 97.5	$ 122.3	$ 126.1	$ 64.0	$ 124.7	$ 129.0	$ 122.1	$ 155.3	$ 409.9	$ 531.1	$ 190.1
Reclassification Adjustment for Amounts Included in Net Income:
Reclassification Adjustment for Amounts Included in Net Income									16.8	28.1	(21.9)
Settlement Expense									64.1	0.0	0.0
Reclassification Adjustments for Amounts Included in Net Income:
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs									70.2	(7.8)	(6.9)
Foreign Currency Translation Adjustments											0.3
Changes in Gain (Loss) on Cash Flow Hedges									0.4	0.4	1.2
Other Comprehensive Income (Loss) Before Income Taxes									437.9	397.9	(241.5)
Reclassification Adjustment for Amounts Included in Net Income:
Reclassification Adjustment for Amounts Included in Net Income									3.6	5.8	4.6
Unrealized Depreciation (Appreciation) on Securities									(78.3)	(85.2)	49.6
Pension Settlement Cost Recognized									(13.5)	0.0	0.0
Reclassification Adjustments for Amounts Included in Net Income:
Net Unrecognized Postretirement Benefit Costs									(15.3)	1.7	1.5
Foreign Currency Translation Adjustments on Investments									0.0	0.0	(0.1)
Changes in Gain (Loss) on Cash Flow Hedges									0.0	(0.1)	(0.3)
Other Comprehensive Income Tax Benefit (Expense)									(93.5)	(83.6)	50.7
Other Comprehensive Income (Loss)									344.4	314.3	(190.8)
Total Comprehensive Income (Loss)									754.3	845.4	(0.7)
No Credit Losses Recognized in Consolidated Statements of Income
Changes in Net Unrealized Gains (Losses) on Investment Securities:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment									386.7	433.4	(214.2)
Reclassification Adjustment for Amounts Included in Net Income:
Changes in Net Unrealized Holding Gains (Losses) on Investment Securities									369.9	405.3	(236.1)
Reclassification Adjustments for Amounts Included in Net Income:
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax									(82.3)	(91.0)	45.0
Credit Losses Recognized in Consolidated Statements of Income
Changes in Net Unrealized Gains (Losses) on Investment Securities:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment									(2.6)	0.0	0.0
Reclassification Adjustment for Amounts Included in Net Income:
Changes in Net Unrealized Holding Gains (Losses) on Investment Securities									(2.6)
Reclassification Adjustments for Amounts Included in Net Income:
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax									0.5	0.0	0.0
Subsidiaries
Reclassification Adjustment for Amounts Included in Net Income:
Reclassification Adjustment for Amounts Included in Net Income									(16.6)	(27.9)	(21.9)
Unrecognized Postretirement Benefit Costs Arising During the Year									0.0	(0.6)	0.0
Reclassification Adjustments for Amounts Included in Net Income:
Foreign Currency Translation Adjustments									0.0	0.0	0.3
Reclassification Adjustment for Amounts Included in Net Income:
Reclassification Adjustment for Amounts Included in Net Income									3.5	5.8	4.6
Reclassification Adjustments for Amounts Included in Net Income:
Foreign Currency Translation Adjustments on Investments									0.0	0.0	(0.1)
Subsidiaries | No Credit Losses Recognized in Consolidated Statements of Income
Changes in Net Unrealized Gains (Losses) on Investment Securities:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment									378.7	433.2	(214.1)
Reclassification Adjustments for Amounts Included in Net Income:
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax									(80.6)	(91.0)	45.0
Subsidiaries | Credit Losses Recognized in Consolidated Statements of Income
Changes in Net Unrealized Gains (Losses) on Investment Securities:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment									(2.6)	0.0	0.0
Reclassification Adjustments for Amounts Included in Net Income:
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax									0.5	0.0	0.0
Parent Company
Condensed Statement of Income Captions [Line Items]
Net Income									409.9	531.1	190.1
Reclassification Adjustment for Amounts Included in Net Income:
Reclassification Adjustment for Amounts Included in Net Income									(0.1)	(0.2)	0.0
Unrecognized Postretirement Benefit Costs Arising During the Year									3.6	(4.2)	(8.0)
Settlement Expense									64.1	0.0	0.0
Reclassification Adjustments for Amounts Included in Net Income:
Amortization of Unrecognized Postretirement Benefits (Costs)									2.5	(3.0)	1.1
Total Reclassification Adjustments for Amounts Included in Net Income									66.6	(3.0)	1.1
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs									70.2	(7.8)	(6.9)
Changes in Gain (Loss) on Cash Flow Hedges									0.4	0.4	1.2
Other Comprehensive Income (Loss) Before Income Taxes									438.0	397.9	(241.5)
Reclassification Adjustment for Amounts Included in Net Income:
Unrecognized Postretirement Benefit Costs Arising During the Year									(1.3)	1.0	1.7
Reclassification Adjustments for Amounts Included in Net Income:
Amortization of Unrecognized Postretirement Benefit Costs									(0.5)	0.7	(0.2)
Net Unrecognized Postretirement Benefit Costs									(15.3)	1.7	1.5
Changes in Gain (Loss) on Cash Flow Hedges									0.0	(0.1)	(0.3)
Other Comprehensive Income Tax Benefit (Expense)									(93.6)	(83.6)	50.7
Other Comprehensive Income (Loss)									344.4	314.3	(190.8)
Total Comprehensive Income (Loss)									754.3	845.4	(0.7)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax									(14.0)	0.7	(0.2)
Parent Company | No Credit Losses Recognized in Consolidated Statements of Income
Changes in Net Unrealized Gains (Losses) on Investment Securities:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment									8.0	0.2	(0.1)
Reclassification Adjustments for Amounts Included in Net Income:
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax									$ (1.7)	$ 0.0	$ 0.0