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, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 3,875.2	$ 1,333.2
Securities, less than 12 months, gross unrealized losses	288.5	12.8
Securities, 12 months or more, fair value	513.0	998.0
Securities, 12 months or more, gross unrealized losses	87.3	20.0
Total, fair value	4,388.2	2,331.2
Total, gross unrealized losses	375.8	32.8
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	8.7	120.3
Securities, less than 12 months, gross unrealized losses	0.2	2.0
Securities, 12 months or more, fair value		99.7
Securities, 12 months or more, gross unrealized losses		0.8
Total, fair value	8.7	220.0
Total, gross unrealized losses	0.2	2.8
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	129.4	90.2
Securities, less than 12 months, gross unrealized losses	2.9	1.3
Securities, 12 months or more, fair value	0.1	4.3
Total, fair value	129.5	94.5
Total, gross unrealized losses	2.9	1.3
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	76.5	198.5
Securities, less than 12 months, gross unrealized losses	0.7	1.2
Securities, 12 months or more, fair value		33.8
Securities, 12 months or more, gross unrealized losses		0.4
Total, fair value	76.5	232.3
Total, gross unrealized losses	0.7	1.6
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,023.5	113.8
Securities, less than 12 months, gross unrealized losses	68.5	1.6
Securities, 12 months or more, fair value	3.6	27.6
Securities, 12 months or more, gross unrealized losses	0.6	0.7
Total, fair value	1,027.1	141.4
Total, gross unrealized losses	69.1	2.3
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	656.5	212.0
Securities, less than 12 months, gross unrealized losses	32.6	2.3
Securities, 12 months or more, fair value	17.4	66.9
Securities, 12 months or more, gross unrealized losses	0.1	0.6
Total, fair value	673.9	278.9
Total, gross unrealized losses	32.7	2.9
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	15.2	48.7
Securities, less than 12 months, gross unrealized losses	0.2	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	15.6	97.3
Total, gross unrealized losses	0.2	0.8
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	436.4	126.8
Securities, less than 12 months, gross unrealized losses	43.9	1.4
Securities, 12 months or more, fair value	0.2	1.2
Total, fair value	436.6	128.0
Total, gross unrealized losses	43.9	1.4
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,529.0	422.9
Securities, less than 12 months, gross unrealized losses	139.5	2.7
Securities, 12 months or more, fair value	491.3	715.9
Securities, 12 months or more, gross unrealized losses	86.6	17.0
Total, fair value	2,020.3	1,138.8
Total, gross unrealized losses	$ 226.1	$ 19.7