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	Dec. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 5,968.9	$ 1,463.8
Securities, less than 12 months, gross unrealized losses	75.4	27.8
Securities, 12 months or more, fair value	695.1	560.0
Securities, 12 months or more, gross unrealized losses	23.4	11.8
Total, fair value	6,664.0	2,023.8
Total, gross unrealized losses	98.8	39.6
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,196.3	255.9
Securities, less than 12 months, gross unrealized losses	11.1	1.5
Securities, 12 months or more, fair value	122.1	0.0
Securities, 12 months or more, gross unrealized losses	5.8	0.0
Total, fair value	1,318.4	255.9
Total, gross unrealized losses	16.9	1.5
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	267.3	36.2
Securities, less than 12 months, gross unrealized losses	4.0	0.6
Securities, 12 months or more, fair value	2.8	0.0
Securities, 12 months or more, gross unrealized losses	0.1	0.0
Total, fair value	270.1	36.2
Total, gross unrealized losses	4.1	0.6
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	350.0	132.8
Securities, less than 12 months, gross unrealized losses	5.3	0.5
Securities, 12 months or more, fair value	73.2	0.0
Securities, 12 months or more, gross unrealized losses	2.6	0.0
Total, fair value	423.2	132.8
Total, gross unrealized losses	7.9	0.5
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	814.2	168.5
Securities, less than 12 months, gross unrealized losses	12.4	3.5
Securities, 12 months or more, fair value	79.3	2.4
Securities, 12 months or more, gross unrealized losses	5.9	0.0
Total, fair value	893.5	170.9
Total, gross unrealized losses	18.3	3.5
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	460.6	36.4
Securities, less than 12 months, gross unrealized losses	8.3	0.2
Securities, 12 months or more, fair value	34.6	19.5
Securities, 12 months or more, gross unrealized losses	1.3	0.5
Total, fair value	495.2	55.9
Total, gross unrealized losses	9.6	0.7
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,072.8	152.6
Securities, less than 12 months, gross unrealized losses	20.5	0.5
Securities, 12 months or more, fair value	50.7	0.4
Securities, 12 months or more, gross unrealized losses	1.3	0.0
Total, fair value	1,123.5	153.0
Total, gross unrealized losses	21.8	0.5
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	197.8	133.5
Securities, less than 12 months, gross unrealized losses	0.6	7.6
Securities, 12 months or more, fair value	40.9	3.8
Securities, 12 months or more, gross unrealized losses	1.6	0.0
Total, fair value	238.7	137.3
Total, gross unrealized losses	2.2	7.6
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,609.9	547.9
Securities, less than 12 months, gross unrealized losses	13.2	13.4
Securities, 12 months or more, fair value	291.5	533.9
Securities, 12 months or more, gross unrealized losses	4.8	11.3
Total, fair value	1,901.4	1,081.8
Total, gross unrealized losses	$ 18.0	$ 24.7