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	Sep. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 2,103.8	$ 1,333.2
Securities, less than 12 months, gross unrealized losses	55.0	12.8
Securities, 12 months or more, fair value	557.9	998.0
Securities, 12 months or more, gross unrealized losses	22.7	20.0
Total, fair value	2,661.7	2,331.2
Total, gross unrealized losses	77.7	32.8
U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	153.3	120.3
Securities, less than 12 months, gross unrealized losses	1.0	2.0
Securities, 12 months or more, fair value	0.0	99.7
Securities, 12 months or more, gross unrealized losses	0.0	0.8
Total, fair value	153.3	220.0
Total, gross unrealized losses	1.0	2.8
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	205.2	90.2
Securities, less than 12 months, gross unrealized losses	3.5	1.3
Securities, 12 months or more, fair value	0.0	4.3
Securities, 12 months or more, gross unrealized losses	0.0	0.0
Total, fair value	205.2	94.5
Total, gross unrealized losses	3.5	1.3
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	110.7	198.5
Securities, less than 12 months, gross unrealized losses	0.4	1.2
Securities, 12 months or more, fair value	0.1	33.8
Securities, 12 months or more, gross unrealized losses	0.0	0.4
Total, fair value	110.8	232.3
Total, gross unrealized losses	0.4	1.6
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	343.3	113.8
Securities, less than 12 months, gross unrealized losses	9.5	1.6
Securities, 12 months or more, fair value	4.6	27.6
Securities, 12 months or more, gross unrealized losses	0.1	0.7
Total, fair value	347.9	141.4
Total, gross unrealized losses	9.6	2.3
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	131.5	212.0
Securities, less than 12 months, gross unrealized losses	1.4	2.3
Securities, 12 months or more, fair value	20.3	66.9
Securities, 12 months or more, gross unrealized losses	1.0	0.6
Total, fair value	151.8	278.9
Total, gross unrealized losses	2.4	2.9
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	289.4	48.7
Securities, less than 12 months, gross unrealized losses	1.0	0.3
Securities, 12 months or more, fair value	0.4	48.6
Securities, 12 months or more, gross unrealized losses	0.0	0.5
Total, fair value	289.8	97.3
Total, gross unrealized losses	1.0	0.8
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	291.5	126.8
Securities, less than 12 months, gross unrealized losses	12.2	1.4
Securities, 12 months or more, fair value	0.3	1.2
Securities, 12 months or more, gross unrealized losses	0.0	0.0
Total, fair value	291.8	128.0
Total, gross unrealized losses	12.2	1.4
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	578.9	422.9
Securities, less than 12 months, gross unrealized losses	26.0	2.7
Securities, 12 months or more, fair value	532.2	715.9
Securities, 12 months or more, gross unrealized losses	21.6	17.0
Total, fair value	1,111.1	1,138.8
Total, gross unrealized losses	$ 47.6	$ 19.7