Gross Unrealized Losses and Related Fair Values for Equity Securities and Debt Securities Grouped by Duration of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 4,180.6
Securities, less than 12 months, gross unrealized losses	97.0
Securities, 12 months or more, fair value	2,265.5
Securities, 12 months or more, gross unrealized losses	73.8
Total, fair value	6,446.1
Total, gross unrealized losses	170.8
Securities, less than 12 months, fair value		$ 2,749.1
Securities, less than 12 months, gross unrealized losses		24.5
Securities, 12 months or more, fair value		1,354.7
Securities, 12 months or more, gross unrealized losses		32.6
Total, fair value		4,103.8
Total, gross unrealized losses		57.1
Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value		145.7
Securities, less than 12 months, gross unrealized losses		3.8
Total, fair value		145.7
Total, gross unrealized losses		3.8
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value		145.7
Securities, less than 12 months, gross unrealized losses		3.8
Total, fair value		145.7
Total, gross unrealized losses		3.8
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	78.5
Securities, less than 12 months, gross unrealized losses	0.9
Securities, 12 months or more, fair value	690.5
Securities, 12 months or more, gross unrealized losses	21.5
Total, fair value	769.0
Total, gross unrealized losses	22.4
Securities, less than 12 months, fair value		447.8
Securities, less than 12 months, gross unrealized losses		4.4
Securities, 12 months or more, fair value		416.6
Securities, 12 months or more, gross unrealized losses		13.2
Total, fair value		864.4
Total, gross unrealized losses		17.6
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	312.4
Securities, less than 12 months, gross unrealized losses	2.5
Securities, 12 months or more, fair value	202.5
Securities, 12 months or more, gross unrealized losses	4.7
Total, fair value	514.9
Total, gross unrealized losses	7.2
Securities, less than 12 months, fair value		240.0
Securities, less than 12 months, gross unrealized losses		1.5
Securities, 12 months or more, fair value		267.3
Securities, 12 months or more, gross unrealized losses		5.1
Total, fair value		507.3
Total, gross unrealized losses		6.6
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	60.7
Securities, less than 12 months, gross unrealized losses	0.1
Securities, 12 months or more, fair value	186.7
Securities, 12 months or more, gross unrealized losses	3.3
Total, fair value	247.4
Total, gross unrealized losses	3.4
Securities, less than 12 months, fair value		321.9
Securities, less than 12 months, gross unrealized losses		2.7
Securities, 12 months or more, fair value		72.2
Securities, 12 months or more, gross unrealized losses		2.0
Total, fair value		394.1
Total, gross unrealized losses		4.7
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,187.9
Securities, less than 12 months, gross unrealized losses	39.4
Securities, 12 months or more, fair value	379.7
Securities, 12 months or more, gross unrealized losses	19.9
Total, fair value	1,567.6
Total, gross unrealized losses	59.3
Securities, less than 12 months, fair value		568.8
Securities, less than 12 months, gross unrealized losses		6.1
Securities, 12 months or more, fair value		207.3
Securities, 12 months or more, gross unrealized losses		3.6
Total, fair value		776.1
Total, gross unrealized losses		9.7
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	501.5
Securities, less than 12 months, gross unrealized losses	9.7
Securities, 12 months or more, fair value	349.1
Securities, 12 months or more, gross unrealized losses	9.1
Total, fair value	850.6
Total, gross unrealized losses	18.8
Securities, less than 12 months, fair value		417.4
Securities, less than 12 months, gross unrealized losses		3.0
Securities, 12 months or more, fair value		159.4
Securities, 12 months or more, gross unrealized losses		3.5
Total, fair value		576.8
Total, gross unrealized losses		6.5
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	397.7
Securities, less than 12 months, gross unrealized losses	6.4
Securities, 12 months or more, fair value	225.9
Securities, 12 months or more, gross unrealized losses	8.4
Total, fair value	623.6
Total, gross unrealized losses	14.8
Securities, less than 12 months, fair value		284.2
Securities, less than 12 months, gross unrealized losses		1.6
Securities, 12 months or more, fair value		131.5
Securities, 12 months or more, gross unrealized losses		3.0
Total, fair value		415.7
Total, gross unrealized losses		4.6
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	199.1
Securities, less than 12 months, gross unrealized losses	1.3
Securities, 12 months or more, fair value	109.5
Securities, 12 months or more, gross unrealized losses	5.1
Total, fair value	308.6
Total, gross unrealized losses	6.4
Securities, less than 12 months, fair value		112.2
Securities, less than 12 months, gross unrealized losses		0.5
Securities, 12 months or more, fair value		34.7
Securities, 12 months or more, gross unrealized losses		1.8
Total, fair value		146.9
Total, gross unrealized losses		2.3
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,442.8
Securities, less than 12 months, gross unrealized losses	36.7
Securities, 12 months or more, fair value	121.6
Securities, 12 months or more, gross unrealized losses	1.8
Total, fair value	1,564.4
Total, gross unrealized losses	$ 38.5
Securities, less than 12 months, fair value		211.1
Securities, less than 12 months, gross unrealized losses		0.9
Securities, 12 months or more, fair value		65.7
Securities, 12 months or more, gross unrealized losses		0.4
Total, fair value		276.8
Total, gross unrealized losses		1.3
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value		2,603.4
Securities, less than 12 months, gross unrealized losses		20.7
Securities, 12 months or more, fair value		1,354.7
Securities, 12 months or more, gross unrealized losses		32.6
Total, fair value		3,958.1
Total, gross unrealized losses		$ 53.3