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	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 872.9	$ 4,180.6
Securities, less than 12 months, gross unrealized losses	9.6	97.0
Securities, 12 months or more, fair value	1,420.8	2,265.5
Securities, 12 months or more, gross unrealized losses	19.2	73.8
Total, fair value	2,293.7	6,446.1
Total, gross unrealized losses	28.8	170.8
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	40.7	78.5
Securities, less than 12 months, gross unrealized losses	0.1	0.9
Securities, 12 months or more, fair value	336.0	690.5
Securities, 12 months or more, gross unrealized losses	2.9	21.5
Total, fair value	376.7	769.0
Total, gross unrealized losses	3.0	22.4
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	2.2	312.4
Securities, less than 12 months, gross unrealized losses		2.5
Securities, 12 months or more, fair value	23.2	202.5
Securities, 12 months or more, gross unrealized losses	0.1	4.7
Total, fair value	25.4	514.9
Total, gross unrealized losses	0.1	7.2
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	33.5	60.7
Securities, less than 12 months, gross unrealized losses		0.1
Securities, 12 months or more, fair value	76.7	186.7
Securities, 12 months or more, gross unrealized losses	0.5	3.3
Total, fair value	110.2	247.4
Total, gross unrealized losses	0.5	3.4
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	121.1	1,187.9
Securities, less than 12 months, gross unrealized losses	2.6	39.4
Securities, 12 months or more, fair value	180.9	379.7
Securities, 12 months or more, gross unrealized losses	2.7	19.9
Total, fair value	302.0	1,567.6
Total, gross unrealized losses	5.3	59.3
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	49.7	501.5
Securities, less than 12 months, gross unrealized losses	0.4	9.7
Securities, 12 months or more, fair value	167.8	349.1
Securities, 12 months or more, gross unrealized losses	1.4	9.1
Total, fair value	217.5	850.6
Total, gross unrealized losses	1.8	18.8
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	40.9	397.7
Securities, less than 12 months, gross unrealized losses	0.5	6.4
Securities, 12 months or more, fair value	212.1	225.9
Securities, 12 months or more, gross unrealized losses	2.5	8.4
Total, fair value	253.0	623.6
Total, gross unrealized losses	3.0	14.8
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	18.0	199.1
Securities, less than 12 months, gross unrealized losses		1.3
Securities, 12 months or more, fair value	19.4	109.5
Securities, 12 months or more, gross unrealized losses	0.6	5.1
Total, fair value	37.4	308.6
Total, gross unrealized losses	0.6	6.4
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	566.8	1,442.8
Securities, less than 12 months, gross unrealized losses	6.0	36.7
Securities, 12 months or more, fair value	404.7	121.6
Securities, 12 months or more, gross unrealized losses	8.5	1.8
Total, fair value	971.5	1,564.4
Total, gross unrealized losses	$ 14.5	$ 38.5