Gross Unrealized Losses and Related Fair Values for AFS Securities for which an Allowance for Credit Losses has Not Been Recorded Grouped by Duration of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 3,341.9	$ 1,463.8
Securities, less than 12 months, gross unrealized losses	82.8	27.8
Securities, 12 months or more, fair value	611.7	560.0
Securities, 12 months or more, gross unrealized losses	14.1	11.8
Total, fair value	3,953.6	2,023.8
Total, gross unrealized losses	96.9	39.6
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	693.9	255.9
Securities, less than 12 months, gross unrealized losses	15.9	1.5
Securities, 12 months or more, fair value	0.0	0.0
Securities, 12 months or more, gross unrealized losses	0.0	0.0
Total, fair value	693.9	255.9
Total, gross unrealized losses	15.9	1.5
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	349.0	36.2
Securities, less than 12 months, gross unrealized losses	9.4	0.6
Securities, 12 months or more, fair value	6.4	0.0
Securities, 12 months or more, gross unrealized losses	0.2	0.0
Total, fair value	355.4	36.2
Total, gross unrealized losses	9.6	0.6
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	307.9	132.8
Securities, less than 12 months, gross unrealized losses	6.8	0.5
Securities, 12 months or more, fair value	10.3	0.0
Securities, 12 months or more, gross unrealized losses	0.1	0.0
Total, fair value	318.2	132.8
Total, gross unrealized losses	6.9	0.5
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	465.3	168.5
Securities, less than 12 months, gross unrealized losses	23.0	3.5
Securities, 12 months or more, fair value	16.5	2.4
Securities, 12 months or more, gross unrealized losses	0.2	0.0
Total, fair value	481.8	170.9
Total, gross unrealized losses	23.2	3.5
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	266.2	36.4
Securities, less than 12 months, gross unrealized losses	5.4	0.2
Securities, 12 months or more, fair value	24.9	19.5
Securities, 12 months or more, gross unrealized losses	0.5	0.5
Total, fair value	291.1	55.9
Total, gross unrealized losses	5.9	0.7
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	890.1	152.6
Securities, less than 12 months, gross unrealized losses	19.4	0.5
Securities, 12 months or more, fair value	0.4	0.4
Securities, 12 months or more, gross unrealized losses	0.0	0.0
Total, fair value	890.5	153.0
Total, gross unrealized losses	19.4	0.5
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	71.7	133.5
Securities, less than 12 months, gross unrealized losses	1.3	7.6
Securities, 12 months or more, fair value	48.9	3.8
Securities, 12 months or more, gross unrealized losses	5.5	0.0
Total, fair value	120.6	137.3
Total, gross unrealized losses	6.8	7.6
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	297.8	547.9
Securities, less than 12 months, gross unrealized losses	1.6	13.4
Securities, 12 months or more, fair value	504.3	533.9
Securities, 12 months or more, gross unrealized losses	7.6	11.3
Total, fair value	802.1	1,081.8
Total, gross unrealized losses	$ 9.2	$ 24.7