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Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]          
AOCI before Tax, Attributable to Parent, beginning balance $ 1,237.3 $ 573.6 $ 435.7 $ (153.0) $ (153.0)
Pretax Reclassification of Stranded Amounts in AOCI to Equity 0.0   3.4    
AOCI before Tax, Attributable to Parent, After Reclassification of Stranded Amounts 1,237.3   439.1    
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment and Tax 163.3 134.3 1,447.7 953.5  
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest 0.0 (1.2) 0.0 (7.4)  
OCI, before Reclassifications, before Tax, Attributable to Parent 163.3 133.1 1,447.7 946.1  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax 194.3 60.7 681.0 147.6  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax (0.3) (0.3) (0.8) (0.8)  
Total reclassification adjustment for amounts realized in net income, pre tax 194.0 60.4 680.2 146.8  
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent (30.7) 72.7 767.5 799.3  
AOCI before Tax, Attributable to Parent, ending balance 1,206.6 646.3 1,206.6 646.3 435.7
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]          
AOCI Tax, Attributable to Parent, beginning balance 262.3 120.5 94.0 (32.1) (32.1)
Tax, Reclassification of Stranded Amounts in AOCI to Equity 0.0   (0.7)    
AOCI Tax, Attributable to Parent, After Reclassification of Stranded Amounts (262.3)   (94.7)    
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax (34.4) (28.2) (304.1) (200.2)  
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest 0.0 0.3 0.0 1.6  
OCI, Debt Securities, Available-for-Sale, Gain (Loss), before Adjustment, Tax (34.4) (27.9) (304.1) (198.6)  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax (40.8) (12.7) (143.0) (30.9)  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax 0.1 0.1 0.2 0.2  
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit (40.7) (12.6) (142.8) (30.7)  
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent 6.3 (15.3) (161.3) (167.9)  
AOCI Tax, Attributable to Parent, ending balance (256.0) (135.8) (256.0) (135.8) 94.0
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]          
After tax total accumulated other comprehensive income, beginning balance 975.0 453.1 341.7 (120.9) (120.9)
Reclassification of Stranded Amounts in AOCI to Equity 0.0   2.7    
AOCI Net of Tax, Attributable to Parent, After Reclassification of Stranded Amounts 975.0   344.4    
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax 128.9 106.1 1,143.6 753.3  
Other comprehensive (income) loss attributable to NCI 0.0 (0.9) 0.0 (5.8) (4.6)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 128.9 105.2 1,143.6 747.5  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax 153.5 48.0 538.0 116.7  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax (0.2) (0.2) (0.6) (0.6)  
Total reclassification adjustment for amounts realized in net income, after tax 153.3 47.8 537.4 116.1  
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent (24.4) 57.4 606.2 631.4  
After tax total accumulated other comprehensive income, ending balance 950.6 510.5 950.6 510.5 341.7
Total net unrealized gains (losses) on securities          
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]          
After tax total accumulated other comprehensive income, beginning balance 991.0 472.9 360.8 (105.6) (105.6)
Reclassification of Stranded Amounts in AOCI to Equity 0.0   0.0    
AOCI Net of Tax, Attributable to Parent, After Reclassification of Stranded Amounts 991.0   360.8    
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax 128.9 106.1 1,143.6 753.3  
Other comprehensive (income) loss attributable to NCI 0.0 0.0 0.0 0.0  
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 128.9 106.1 1,143.6 753.3  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax 153.5 48.0 538.0 116.7  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax 0.0 0.0 0.0 0.0  
Total reclassification adjustment for amounts realized in net income, after tax 153.5 48.0 538.0 116.7  
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent (24.6) 58.1 605.6 636.6  
After tax total accumulated other comprehensive income, ending balance 966.4 531.0 966.4 531.0 360.8
Net unrealized gains (losses) on forecasted transactions          
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]          
After tax total accumulated other comprehensive income, beginning balance (16.0) (16.8) (16.4) (17.2) (17.2)
Reclassification of Stranded Amounts in AOCI to Equity 0.0   0.0    
AOCI Net of Tax, Attributable to Parent, After Reclassification of Stranded Amounts (16.0)   (16.4)    
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax 0.0 0.0 0.0 0.0  
Other comprehensive (income) loss attributable to NCI 0.0 0.0 0.0 0.0  
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 0.0 0.0 0.0 0.0  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax 0.0 0.0 0.0 0.0  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax (0.2) (0.2) (0.6) (0.6)  
Total reclassification adjustment for amounts realized in net income, after tax (0.2) (0.2) (0.6) (0.6)  
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent 0.2 0.2 0.6 0.6  
After tax total accumulated other comprehensive income, ending balance (15.8) (16.6) (15.8) (16.6) (16.4)
(Income) loss attributable to NCI          
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]          
After tax total accumulated other comprehensive income, beginning balance 0.0 (3.0) (2.7) 1.9 1.9
Reclassification of Stranded Amounts in AOCI to Equity 0.0   2.7    
AOCI Net of Tax, Attributable to Parent, After Reclassification of Stranded Amounts 0.0   0.0    
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax 0.0 0.0 0.0 0.0  
Other comprehensive (income) loss attributable to NCI 0.0 (0.9) 0.0 (5.8)  
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 0.0 (0.9) 0.0 (5.8)  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax 0.0 0.0 0.0 0.0  
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax 0.0 0.0 0.0 0.0  
Total reclassification adjustment for amounts realized in net income, after tax 0.0 0.0 0.0 0.0  
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent 0.0 (0.9) 0.0 (5.8)  
After tax total accumulated other comprehensive income, ending balance $ 0.0 $ (3.9) $ 0.0 $ (3.9) $ (2.7)