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	Jun. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 2,248.9	$ 1,333.2
Securities, less than 12 months, gross unrealized losses	89.6	12.8
Securities, 12 months or more, fair value	526.0	998.0
Securities, 12 months or more, gross unrealized losses	33.4	20.0
Total, fair value	2,774.9	2,331.2
Total, gross unrealized losses	123.0	32.8
U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	110.7	120.3
Securities, less than 12 months, gross unrealized losses	0.3	2.0
Securities, 12 months or more, fair value		99.7
Securities, 12 months or more, gross unrealized losses		0.8
Total, fair value	110.7	220.0
Total, gross unrealized losses	0.3	2.8
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	95.2	90.2
Securities, less than 12 months, gross unrealized losses	1.8	1.3
Securities, 12 months or more, fair value		4.3
Total, fair value	95.2	94.5
Total, gross unrealized losses	1.8	1.3
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	92.1	198.5
Securities, less than 12 months, gross unrealized losses	0.5	1.2
Securities, 12 months or more, fair value		33.8
Securities, 12 months or more, gross unrealized losses		0.4
Total, fair value	92.1	232.3
Total, gross unrealized losses	0.5	1.6
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	373.6	113.8
Securities, less than 12 months, gross unrealized losses	9.2	1.6
Securities, 12 months or more, fair value	4.2	27.6
Securities, 12 months or more, gross unrealized losses	0.3	0.7
Total, fair value	377.8	141.4
Total, gross unrealized losses	9.5	2.3
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	252.4	212.0
Securities, less than 12 months, gross unrealized losses	6.0	2.3
Securities, 12 months or more, fair value	0.9	66.9
Securities, 12 months or more, gross unrealized losses		0.6
Total, fair value	253.3	278.9
Total, gross unrealized losses	6.0	2.9
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	307.9	48.7
Securities, less than 12 months, gross unrealized losses	0.6	0.3
Securities, 12 months or more, fair value	0.4	48.6
Securities, 12 months or more, gross unrealized losses		0.5
Total, fair value	308.3	97.3
Total, gross unrealized losses	0.6	0.8
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	332.3	126.8
Securities, less than 12 months, gross unrealized losses	25.0	1.4
Securities, 12 months or more, fair value	0.2	1.2
Total, fair value	332.5	128.0
Total, gross unrealized losses	25.0	1.4
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	684.7	422.9
Securities, less than 12 months, gross unrealized losses	46.2	2.7
Securities, 12 months or more, fair value	520.3	715.9
Securities, 12 months or more, gross unrealized losses	33.1	17.0
Total, fair value	1,205.0	1,138.8
Total, gross unrealized losses	$ 79.3	$ 19.7