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Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]        
Balance at Beginning of Period     $ 3,972.3  
Balance at End of Period $ 4,347.5   4,347.5  
Reclassification Adjustment for Amounts Included in Net Income        
Reclassification Adjustment for Amounts Included in Net Income (9.0) $ 0.2 (16.4) $ (26.9)
Income Tax Benefit (Expense) 1.9 0.0 3.4 5.7
Net of Taxes (7.1) 0.2 (13.0) (21.2)
Changes in Net Unrecognized Postretirement Benefit Costs        
Net Unrecognized Postretirement Benefit Costs 0.9 (0.4) 2.3 0.2
Income Tax Benefit (Expense) (0.2) 0.0 (0.5) (0.1)
Net of Taxes 0.7 (0.4) 1.8 0.1
Changes in Gain (Loss) on Cash Flow Hedges        
Gain on Cash Flow Hedges 0.2 0.2 0.3 0.3
Income Tax Benefit (Expense) (0.1) (0.1) (0.1) (0.1)
Net of Taxes 0.1 0.1 0.2 0.2
Total Other Comprehensive Income (Loss) Before Income Taxes 65.5 118.1 269.5 440.9
Total Income Tax Benefit (Expense) (13.8) (24.9) (56.6) (92.6)
Other Comprehensive Income (Loss), Net of Taxes 51.7 93.2 212.9 348.3
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 72.1 118.1 285.7 467.3
Income Tax Benefit (Expense) (15.1) (24.8) (59.9) (98.1)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax 57.0 93.3 225.8 369.2
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 1.3 0.0 (2.4) 0.0
Income Tax Benefit (Expense) (0.3) 0.0 0.5 0.0
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax 1.0 0.0 (1.9) 0.0
Having No Credit Losses Recognized in Consolidated Statements of Income        
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]        
Balance at Beginning of Period 602.3   439.4  
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax 57.0   225.8  
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax (7.1)   (13.0)  
Other Comprehensive Income (Loss), Net of Tax 49.9   212.8  
Balance at End of Period 652.2   652.2  
Having Credit Losses Recognized in Consolidated Statements of Income        
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]        
Balance at Beginning of Period (2.9)   0.0  
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax 1.0   (1.9)  
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax 0.0   0.0  
Other Comprehensive Income (Loss), Net of Tax 1.0   (1.9)  
Balance at End of Period (1.9)   (1.9)  
Net Unrecognized Postretirement Benefit Costs        
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]        
Balance at Beginning of Period (99.5)   (100.6)  
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax 0.7   1.8  
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax 0.0   0.0  
Other Comprehensive Income (Loss), Net of Tax 0.7   1.8  
Balance at End of Period (98.8)   (98.8)  
Gain (Loss) on Cash Flow Hedges        
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]        
Balance at Beginning of Period (2.6)   (2.7)  
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax 0.1   0.2  
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax 0.0   0.0  
Other Comprehensive Income (Loss), Net of Tax 0.1   0.2  
Balance at End of Period (2.5)   (2.5)  
Accumulated Other Comprehensive Income        
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]        
Balance at Beginning of Period 497.3   336.1  
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax 58.8   225.9  
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax (7.1)   (13.0)  
Other Comprehensive Income (Loss), Net of Tax 51.7   212.9  
Balance at End of Period 549.0   549.0  
Changes in Gain (Loss) on Cash Flow Hedges        
Other Comprehensive Income (Loss), Net of Taxes $ 51.7 $ 93.2 $ 212.9 $ 348.3