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	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 1,463.8	$ 1,333.2
Securities, less than 12 months, gross unrealized losses	27.8	12.8
Securities, 12 months or more, fair value	560.0	998.0
Securities, 12 months or more, gross unrealized losses	11.8	20.0
Total, fair value	2,023.8	2,331.2
Total, gross unrealized losses	39.6	32.8
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	255.9	120.3
Securities, less than 12 months, gross unrealized losses	1.5	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	255.9	220.0
Total, gross unrealized losses	1.5	2.8
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	36.2	90.2
Securities, less than 12 months, gross unrealized losses	0.6	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	36.2	94.5
Total, gross unrealized losses	0.6	1.3
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	132.8	198.5
Securities, less than 12 months, gross unrealized losses	0.5	1.2
Securities, 12 months or more, fair value	0.0	33.8
Securities, 12 months or more, gross unrealized losses	0.0	0.4
Total, fair value	132.8	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	168.5	113.8
Securities, less than 12 months, gross unrealized losses	3.5	1.6
Securities, 12 months or more, fair value	2.4	27.6
Securities, 12 months or more, gross unrealized losses	0.0	0.7
Total, fair value	170.9	141.4
Total, gross unrealized losses	3.5	2.3
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	36.4	212.0
Securities, less than 12 months, gross unrealized losses	0.2	2.3
Securities, 12 months or more, fair value	19.5	66.9
Securities, 12 months or more, gross unrealized losses	0.5	0.6
Total, fair value	55.9	278.9
Total, gross unrealized losses	0.7	2.9
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	152.6	48.7
Securities, less than 12 months, gross unrealized losses	0.5	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	153.0	97.3
Total, gross unrealized losses	0.5	0.8
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	133.5	126.8
Securities, less than 12 months, gross unrealized losses	7.6	1.4
Securities, 12 months or more, fair value	3.8	1.2
Securities, 12 months or more, gross unrealized losses	0.0	0.0
Total, fair value	137.3	128.0
Total, gross unrealized losses	7.6	1.4
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	547.9	422.9
Securities, less than 12 months, gross unrealized losses	13.4	2.7
Securities, 12 months or more, fair value	533.9	715.9
Securities, 12 months or more, gross unrealized losses	11.3	17.0
Total, fair value	1,081.8	1,138.8
Total, gross unrealized losses	$ 24.7	$ 19.7