Gross Unrealized Losses and Related Fair Values for AFS Securities before an Allowance for Credit Losses Grouped by Duration of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 9,227.9	$ 5,968.9
Securities, less than 12 months, gross unrealized losses	408.3	75.4
Securities, 12 months or more, fair value	1,555.0	695.1
Securities, 12 months or more, gross unrealized losses	131.4	23.4
Total, fair value	10,782.9	6,664.0
Total, gross unrealized losses	539.7	98.8
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,224.2	1,196.3
Securities, less than 12 months, gross unrealized losses	50.4	11.1
Securities, 12 months or more, fair value	149.6	122.1
Securities, 12 months or more, gross unrealized losses	10.5	5.8
Total, fair value	1,373.8	1,318.4
Total, gross unrealized losses	60.9	16.9
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	966.9	267.3
Securities, less than 12 months, gross unrealized losses	67.4	4.0
Securities, 12 months or more, fair value	25.3	2.8
Securities, 12 months or more, gross unrealized losses	3.1	0.1
Total, fair value	992.2	270.1
Total, gross unrealized losses	70.5	4.1
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	533.2	350.0
Securities, less than 12 months, gross unrealized losses	16.4	5.3
Securities, 12 months or more, fair value	138.6	73.2
Securities, 12 months or more, gross unrealized losses	11.4	2.6
Total, fair value	671.8	423.2
Total, gross unrealized losses	27.8	7.9
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,633.7	814.2
Securities, less than 12 months, gross unrealized losses	85.1	12.4
Securities, 12 months or more, fair value	192.9	79.3
Securities, 12 months or more, gross unrealized losses	29.9	5.9
Total, fair value	1,826.6	893.5
Total, gross unrealized losses	115.0	18.3
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	804.4	460.6
Securities, less than 12 months, gross unrealized losses	37.3	8.3
Securities, 12 months or more, fair value	110.4	34.6
Securities, 12 months or more, gross unrealized losses	11.1	1.3
Total, fair value	914.8	495.2
Total, gross unrealized losses	48.4	9.6
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	879.0	1,072.8
Securities, less than 12 months, gross unrealized losses	48.3	20.5
Securities, 12 months or more, fair value	551.9	50.7
Securities, 12 months or more, gross unrealized losses	50.9	1.3
Total, fair value	1,430.9	1,123.5
Total, gross unrealized losses	99.2	21.8
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	830.9	197.8
Securities, less than 12 months, gross unrealized losses	18.7	0.6
Securities, 12 months or more, fair value	35.4	40.9
Securities, 12 months or more, gross unrealized losses	2.9	1.6
Total, fair value	866.3	238.7
Total, gross unrealized losses	21.6	2.2
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	2,355.6	1,609.9
Securities, less than 12 months, gross unrealized losses	84.7	13.2
Securities, 12 months or more, fair value	350.9	291.5
Securities, 12 months or more, gross unrealized losses	11.6	4.8
Total, fair value	2,706.5	1,901.4
Total, gross unrealized losses	$ 96.3	$ 18.0