Gross Unrealized Losses and Related Fair Values for Available-for-Sale Securities Grouped by Duration of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 845.4	$ 4,180.6
Securities, less than 12 months, gross unrealized losses	10.6	97.0
Securities, 12 months or more, fair value	1,020.7	2,265.5
Securities, 12 months or more, gross unrealized losses	19.2	73.8
Total, fair value	1,866.1	6,446.1
Total, gross unrealized losses	29.8	170.8
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	57.2	78.5
Securities, less than 12 months, gross unrealized losses	0.5	0.9
Securities, 12 months or more, fair value	157.2	690.5
Securities, 12 months or more, gross unrealized losses	1.0	21.5
Total, fair value	214.4	769.0
Total, gross unrealized losses	1.5	22.4
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	4.8	312.4
Securities, less than 12 months, gross unrealized losses		2.5
Securities, 12 months or more, fair value	5.3	202.5
Securities, 12 months or more, gross unrealized losses		4.7
Total, fair value	10.1	514.9
Total, gross unrealized losses		7.2
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	22.0	60.7
Securities, less than 12 months, gross unrealized losses	0.1	0.1
Securities, 12 months or more, fair value	39.2	186.7
Securities, 12 months or more, gross unrealized losses	0.3	3.3
Total, fair value	61.2	247.4
Total, gross unrealized losses	0.4	3.4
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	156.1	1,187.9
Securities, less than 12 months, gross unrealized losses	2.1	39.4
Securities, 12 months or more, fair value	52.5	379.7
Securities, 12 months or more, gross unrealized losses	1.4	19.9
Total, fair value	208.6	1,567.6
Total, gross unrealized losses	3.5	59.3
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	101.8	501.5
Securities, less than 12 months, gross unrealized losses	4.1	9.7
Securities, 12 months or more, fair value	80.6	349.1
Securities, 12 months or more, gross unrealized losses	0.7	9.1
Total, fair value	182.4	850.6
Total, gross unrealized losses	4.8	18.8
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	57.8	397.7
Securities, less than 12 months, gross unrealized losses	0.3	6.4
Securities, 12 months or more, fair value	96.6	225.9
Securities, 12 months or more, gross unrealized losses	0.9	8.4
Total, fair value	154.4	623.6
Total, gross unrealized losses	1.2	14.8
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	67.8	199.1
Securities, less than 12 months, gross unrealized losses	0.1	1.3
Securities, 12 months or more, fair value	1.3	109.5
Securities, 12 months or more, gross unrealized losses		5.1
Total, fair value	69.1	308.6
Total, gross unrealized losses	0.1	6.4
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	377.9	1,442.8
Securities, less than 12 months, gross unrealized losses	3.4	36.7
Securities, 12 months or more, fair value	588.0	121.6
Securities, 12 months or more, gross unrealized losses	14.9	1.8
Total, fair value	965.9	1,564.4
Total, gross unrealized losses	$ 18.3	$ 38.5