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Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Unrealized Gain (Loss) [Abstract]        
Accumulated Other Comprehensive Income $ 448.1 $ 680.5    
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent 59.7 (78.7) $ (85.2)  
Reclassification from AOCI, Current Period, Tax 11.3 3.6 5.8  
Accumulated Net Unrealized Investment Gain (Loss)        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent 0.4 0.5 0.0  
Reclassification from AOCI, Current Period, Tax 0.0 0.0 0.0  
Accumulated Defined Benefit Plans Adjustment        
Debt Securities, Unrealized Gain (Loss) [Abstract]        
Net Unrecognized Postretirement Benefit Costs, Net of Income Taxes       $ (94.5)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent 2.4 (15.3) 1.7  
Reclassification from AOCI, Current Period, Tax 0.0 (13.5) 0.0  
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent (0.1) 0.0 (0.1)  
Reclassification from AOCI, Current Period, Tax (0.1) 0.0 0.0  
Accumulated Other Comprehensive Income (Loss)        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent 62.4 (93.5) (83.6)  
Reclassification from AOCI, Current Period, Tax $ 11.2 $ (9.9) $ 5.8