Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ 1,187.4	$ 435.7
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment and Tax	(551.4)	318.2
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest	0.0	(0.6)
OCI, before Reclassifications, before Tax, Attributable to Parent	(551.4)	317.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	131.7	238.7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(0.3)	(0.3)
Total reclassification adjustment for amounts realized in net income, pre tax	131.4	238.4
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(682.8)	79.2
AOCI before Tax, Attributable to Parent, ending balance	504.6	514.9
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	(255.7)	(94.0)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax	115.8	(66.8)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	0.0	0.1
OCI, Debt Securities, Available-for-Sale, Gain (Loss), before Adjustment, Tax	115.8	(66.7)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	(27.7)	(50.1)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0.1	0.1
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	(27.6)	(50.0)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	143.4	(16.7)
AOCI Tax, Attributable to Parent, ending balance	(112.3)	(110.7)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	931.7	341.7
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(435.6)	251.4
Other comprehensive income attributable to NCI	0.0	(0.5)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(435.6)	250.9
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	104.0	188.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)	(0.2)
Total reclassification adjustment for amounts realized in net income, after tax	103.8	188.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(539.4)	62.5
After tax total accumulated other comprehensive income, ending balance	392.3	404.2
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	947.3	360.8
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(435.6)	251.4
Other comprehensive income attributable to NCI	0.0	0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(435.6)	251.4
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	104.0	188.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0.0	0.0
Total reclassification adjustment for amounts realized in net income, after tax	104.0	188.6
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(539.6)	62.8
After tax total accumulated other comprehensive income, ending balance	407.7	423.6
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	(15.6)	(16.4)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	0.0	0.0
Other comprehensive income attributable to NCI	0.0	0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0.0	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0.0	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)	(0.2)
Total reclassification adjustment for amounts realized in net income, after tax	(0.2)	(0.2)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.2	0.2
After tax total accumulated other comprehensive income, ending balance	(15.4)	(16.2)
(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	(2.7)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	0.0	0.0
Other comprehensive income attributable to NCI	0.0	(0.5)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0.0	(0.5)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0.0	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0.0	0.0
Total reclassification adjustment for amounts realized in net income, after tax	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.0	(0.5)
After tax total accumulated other comprehensive income, ending balance	$ 0.0	$ (3.2)