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Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]      
Balance at Beginning of Period $ 4,563.4    
Balance at End of Period 4,339.1   $ 4,563.4
Reclassification Adjustment for Amounts Included in Net Income      
Reclassification Adjustment for Amounts Included in Net Income (9.8) $ (3.9)  
Income Tax Benefit (Expense) 2.1 0.8  
Net of Taxes (7.7) (3.1)  
Changes in Net Unrecognized Postretirement Benefit Costs      
Net Unrecognized Postretirement Benefit Costs 0.3 0.7  
Income Tax Benefit (Expense) 0.5 (0.2)  
Net of Taxes 0.8 0.5  
Changes in Gain (Loss) on Cash Flow Hedges      
Gain on Cash Flow Hedges 0.1 0.1  
Income Tax Benefit (Expense) 0.0 0.0  
Net of Taxes 0.1 0.1  
Total Other Comprehensive Income (Loss) Before Income Taxes (365.7) (204.0)  
Total Income Tax Benefit (Expense) 75.7 42.8  
Other Comprehensive Income (Loss), Net of Taxes (290.0) (161.2)  
Net Income 123.2 64.0  
Income Tax Benefit (Expense) 1.6 (13.7)  
Impairment Losses (4.0) (12.0)  
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax     (45.7)
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax     (2.3)
Reclassification out of Accumulated Other Comprehensive Income      
Changes in Gain (Loss) on Cash Flow Hedges      
Net Income 6.8 2.4  
No Credit Losses Recognized in Condensed Consolidated Statements of Income      
Changes in Net Unrealized Gains (Losses) on Investment Securities:      
Changes in Net Unrealized Gains (Losses) on Investment Securities (354.2) (198.0)  
Income Tax Benefit (Expense) 72.7 41.6  
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax (281.5) (156.4)  
Credit Losses Recognized in Condensed Consolidated Statements of Income      
Changes in Net Unrealized Gains (Losses) on Investment Securities:      
Changes in Net Unrealized Gains (Losses) on Investment Securities (2.1) (2.9)  
Income Tax Benefit (Expense) 0.4 0.6  
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax (1.7) (2.3)  
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]      
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]      
Balance at Beginning of Period 730.6    
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax (281.5)    
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax (7.7)    
Other Comprehensive Income (Loss), Net of Tax (289.2)    
Balance at End of Period 441.4   730.6
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income      
Changes in Gain (Loss) on Cash Flow Hedges      
Net Income 7.7 3.1  
Income Tax Benefit (Expense) (2.1) (0.8)  
Net Realized Gains on Sales of Investments 13.8 15.9  
Impairment Losses (4.0) (12.0)  
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest 9.8 3.9  
Accumulated Other-than-Temporary Impairment Attributable to Parent [Member]      
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]      
Balance at Beginning of Period (2.1)    
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax (1.7)    
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax 0.0    
Other Comprehensive Income (Loss), Net of Tax (1.7)    
Balance at End of Period (3.8)   (2.1)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]      
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]      
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax 0.8    
Other Comprehensive Income (Loss), Net of Tax 0.8    
Balance at End of Period (44.9)    
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income      
Changes in Gain (Loss) on Cash Flow Hedges      
Net Income (0.8) (0.6)  
Interest and Other Expenses (0.3) (0.7)  
Income Tax Benefit (Expense) (0.5) 0.1  
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]      
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]      
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax 0.1    
Other Comprehensive Income (Loss), Net of Tax 0.1    
Balance at End of Period (2.2)    
Accumulated Other Comprehensive Income      
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]      
Balance at Beginning of Period 680.5    
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax (282.3)    
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax (7.7)    
Other Comprehensive Income (Loss), Net of Tax (290.0)    
Balance at End of Period 390.5   $ 680.5
Changes in Gain (Loss) on Cash Flow Hedges      
Other Comprehensive Income (Loss), Net of Taxes $ (290.0) $ (161.2)