Investments - 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	Jun. 30, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 11,087.3	$ 5,968.9
Securities, less than 12 months, gross unrealized losses	840.4	75.4
Securities, 12 months or more, fair value	1,844.3	695.1
Securities, 12 months or more, gross unrealized losses	220.6	23.4
Total, fair value	12,931.6	6,664.0
Total, gross unrealized losses	1,061.0	98.8
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,285.4	1,196.3
Securities, less than 12 months, gross unrealized losses	82.8	11.1
Securities, 12 months or more, fair value	240.7	122.1
Securities, 12 months or more, gross unrealized losses	16.6	5.8
Total, fair value	1,526.1	1,318.4
Total, gross unrealized losses	99.4	16.9
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,543.9	267.3
Securities, less than 12 months, gross unrealized losses	146.8	4.0
Securities, 12 months or more, fair value	24.0	2.8
Securities, 12 months or more, gross unrealized losses	4.4	0.1
Total, fair value	1,567.9	270.1
Total, gross unrealized losses	151.2	4.1
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	572.4	350.0
Securities, less than 12 months, gross unrealized losses	31.3	5.3
Securities, 12 months or more, fair value	130.2	73.2
Securities, 12 months or more, gross unrealized losses	15.0	2.6
Total, fair value	702.6	423.2
Total, gross unrealized losses	46.3	7.9
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	2,569.2	814.2
Securities, less than 12 months, gross unrealized losses	208.8	12.4
Securities, 12 months or more, fair value	187.9	79.3
Securities, 12 months or more, gross unrealized losses	47.8	5.9
Total, fair value	2,757.1	893.5
Total, gross unrealized losses	256.6	18.3
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	914.6	460.6
Securities, less than 12 months, gross unrealized losses	78.1	8.3
Securities, 12 months or more, fair value	102.4	34.6
Securities, 12 months or more, gross unrealized losses	15.9	1.3
Total, fair value	1,017.0	495.2
Total, gross unrealized losses	94.0	9.6
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,137.1	1,072.8
Securities, less than 12 months, gross unrealized losses	92.9	20.5
Securities, 12 months or more, fair value	506.8	50.7
Securities, 12 months or more, gross unrealized losses	79.6	1.3
Total, fair value	1,643.9	1,123.5
Total, gross unrealized losses	172.5	21.8
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	853.8	197.8
Securities, less than 12 months, gross unrealized losses	51.1	0.6
Securities, 12 months or more, fair value	33.8	40.9
Securities, 12 months or more, gross unrealized losses	4.2	1.6
Total, fair value	887.6	238.7
Total, gross unrealized losses	55.3	2.2
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	2,210.9	1,609.9
Securities, less than 12 months, gross unrealized losses	148.6	13.2
Securities, 12 months or more, fair value	618.5	291.5
Securities, 12 months or more, gross unrealized losses	37.1	4.8
Total, fair value	2,829.4	1,901.4
Total, gross unrealized losses	$ 185.7	$ 18.0