Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ (1,753.4)	$ 504.6	$ 52.3	$ 1,187.4
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment and Tax	(1,246.7)	174.7	(3,098.4)	(376.7)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss), before Reclassification and Tax	(0.5)	(0.6)	(0.2)	(0.6)
OCI, before Reclassifications, before Tax, Attributable to Parent	(1,247.2)	174.1	(3,098.6)	(377.3)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	(205.1)	59.4	(250.6)	191.1
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	0.0	(0.3)	(0.2)	(0.6)
Total reclassification adjustment for amounts realized in net income, pre tax	(205.1)	59.1	(250.8)	190.5
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(1,042.1)	115.0	(2,847.8)	(567.8)
AOCI before Tax, Attributable to Parent, ending balance	(2,795.5)	619.6	(2,795.5)	619.6
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	367.6	(112.3)	(11.6)	(255.7)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax	261.8	(36.7)	650.7	79.1
Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax, Portion Attributable to Parent	(0.1)	(0.1)	0.0	(0.1)
OCI, Debt Securities, Available-for-Sale, Gain (Loss), before Adjustment, Tax	261.9	(36.6)	650.7	79.2
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	43.0	(12.5)	52.6	(40.2)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	0.0	0.0	0.0	(0.1)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	43.0	(12.5)	52.6	(40.1)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	218.9	(24.1)	598.1	119.3
AOCI Tax, Attributable to Parent, ending balance	586.5	(136.4)	586.5	(136.4)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(1,385.8)	392.3	40.7	931.7
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(984.9)	138.0	(2,447.7)	(297.6)
Foreign currency translation adjustment	(0.4)	(0.5)	(0.2)	(0.5)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(985.3)	137.5	(2,447.9)	(298.1)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(162.1)	46.9	(198.0)	150.9
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	0.0	(0.3)	(0.2)	(0.5)
Total reclassification adjustment for amounts realized in net income, after tax	(162.1)	46.6	(198.2)	150.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(823.2)	90.9	(2,249.7)	(448.5)
After tax total accumulated other comprehensive income, ending balance	(2,209.0)	483.2	(2,209.0)	483.2
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	(1,370.7)	407.7	56.2	947.3
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(984.9)	138.0	(2,447.7)	(297.6)
Foreign currency translation adjustment	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(984.9)	138.0	(2,447.7)	(297.6)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(162.1)	46.9	(198.0)	150.9
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	0.0	0.0	0.0	0.0
Total reclassification adjustment for amounts realized in net income, after tax	(162.1)	46.9	(198.0)	150.9
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(822.8)	91.1	(2,249.7)	(448.5)
After tax total accumulated other comprehensive income, ending balance	(2,193.5)	498.8	(2,193.5)	498.8
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	(14.7)	(15.4)	(14.9)	(15.6)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment	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), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	0.0	(0.3)	(0.2)	(0.5)
Total reclassification adjustment for amounts realized in net income, after tax	0.0	(0.3)	(0.2)	(0.5)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.0	0.3	0.2	0.5
After tax total accumulated other comprehensive income, ending balance	(14.7)	(15.1)	(14.7)	(15.1)
Accumulated Foreign Currency Adjustment Attributable to Parent
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(0.4)	0.0	(0.6)	0.0
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment	(0.4)	(0.5)	(0.2)	(0.5)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(0.4)	(0.5)	(0.2)	(0.5)
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), Cash Flow Hedge, Gain (Loss), Reclassification, after 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	(0.4)	(0.5)	(0.2)	(0.5)
After tax total accumulated other comprehensive income, ending balance	$ (0.8)	$ (0.5)	$ (0.8)	$ (0.5)