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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ 504.6	$ 514.9	$ 1,187.4	$ 435.7
Pretax Reclassification of Stranded Amounts in AOCI to Equity					$ 4.0	$ 3.4
AOCI before Tax, Attributable to Parent, After Reclassification of Stranded Amounts					518.9	439.1
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment and Tax	174.7	966.2	(376.7)	1,284.4
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss), before Reclassification and Tax	(0.6)	0.0	(0.6)	0.0
OCI, before Reclassifications, before Tax, Attributable to Parent	174.1	966.2	(377.3)	1,284.4
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	59.4	248.0	191.1	486.7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(0.3)	(0.2)	(0.6)	(0.5)
Total reclassification adjustment for amounts realized in net income, pre tax	59.1	247.8	190.5	486.2
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	115.0	718.4	(567.8)	798.2
AOCI before Tax, Attributable to Parent, ending balance	619.6	1,237.3	619.6	1,237.3
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	(112.3)	(110.7)	(255.7)	(94.0)
Tax, Reclassification of Stranded Amounts in AOCI to Equity					(0.8)	(0.7)
AOCI Tax, Attributable to Parent, After Reclassification of Stranded Amounts					(111.5)	(94.7)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax	(36.7)	(202.9)	79.1	(269.7)
Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax, Portion Attributable to Parent	(0.1)	0.0	(0.1)	0.0
OCI, Debt Securities, Available-for-Sale, Gain (Loss), before Adjustment, Tax	(36.6)	(202.9)	79.2	(269.7)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	(12.5)	(52.1)	(40.2)	(102.2)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0.0	0.0	0.1	0.1
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	(12.5)	(52.1)	(40.1)	(102.1)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(24.1)	(150.8)	119.3	(167.6)
AOCI Tax, Attributable to Parent, ending balance	(136.4)	(262.3)	(136.4)	(262.3)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	392.3	404.2	931.7	341.7
Reclassification of Stranded Amounts in AOCI to Equity					3.2	2.7
AOCI Net of Tax, Attributable to Parent, After Reclassification of Stranded Amounts					407.4	344.4
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	138.0	763.3	(297.6)	1,014.7
Foreign currency translation adjustment	(0.5)	0.0	(0.5)	0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	137.5	763.3	(298.1)	1,014.7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	46.9	195.9	150.9	384.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.3)	(0.2)	(0.5)	(0.4)
Total reclassification adjustment for amounts realized in net income, after tax	46.6	195.7	150.4	384.1
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	90.9	567.6	(448.5)	630.6
After tax total accumulated other comprehensive income, ending balance	483.2	975.0	483.2	975.0
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	407.7	423.6	947.3	360.8
Reclassification of Stranded Amounts in AOCI to Equity					0.0	0.0
AOCI Net of Tax, Attributable to Parent, After Reclassification of Stranded Amounts					423.6	360.8
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	138.0	763.3	(297.6)	1,014.7
Foreign currency translation adjustment	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	138.0	763.3	(297.6)	1,014.7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	46.9	195.9	150.9	384.5
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	46.9	195.9	150.9	384.5
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	91.1	567.4	(448.5)	630.2
After tax total accumulated other comprehensive income, ending balance	498.8	991.0	498.8	991.0
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	(15.4)	(16.2)	(15.6)	(16.4)
Reclassification of Stranded Amounts in AOCI to Equity					0.0	0.0
AOCI Net of Tax, Attributable to Parent, After Reclassification of Stranded Amounts					(16.2)	(16.4)
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), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.3)	(0.2)	(0.5)	(0.4)
Total reclassification adjustment for amounts realized in net income, after tax	(0.3)	(0.2)	(0.5)	(0.4)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.3	0.2	0.5	0.4
After tax total accumulated other comprehensive income, ending balance	(15.1)	(16.0)	(15.1)	(16.0)
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	0.0	0.0	0.0	0.0
Reclassification of Stranded Amounts in AOCI to Equity					0.0	0.0
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
Foreign currency translation adjustment	(0.5)	0.0	(0.5)	0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(0.5)	0.0	(0.5)	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	0.0	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	(0.5)	0.0	(0.5)	0.0
After tax total accumulated other comprehensive income, ending balance	(0.5)	0.0	(0.5)	0.0
(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	(3.2)	0.0	(2.7)
Reclassification of Stranded Amounts in AOCI to Equity					3.2	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
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), 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	0.0	0.0	0.0
After tax total accumulated other comprehensive income, ending balance	$ 0.0	$ 0.0	$ 0.0	$ 0.0