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, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 5,491.7	$ 6,557.4
Securities, less than 12 months, gross unrealized losses	146.6	197.5
Securities, 12 months or more, fair value	809.8	729.4
Securities, 12 months or more, gross unrealized losses	17.2	35.3
Total, fair value	6,301.5	7,286.8
Total, gross unrealized losses	163.8	232.8
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	619.4	1,355.6
Securities, less than 12 months, gross unrealized losses	39.2	87.0
Total, fair value	619.4	1,355.6
Total, gross unrealized losses	39.2	87.0
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	619.4	1,355.6
Securities, less than 12 months, gross unrealized losses	39.2	87.0
Total, fair value	619.4	1,355.6
Total, gross unrealized losses	39.2	87.0
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	4,872.3	5,201.8
Securities, less than 12 months, gross unrealized losses	107.4	110.5
Securities, 12 months or more, fair value	809.8	729.4
Securities, 12 months or more, gross unrealized losses	17.2	35.3
Total, fair value	5,682.1	5,931.2
Total, gross unrealized losses	124.6	145.8
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	975.0	818.4
Securities, less than 12 months, gross unrealized losses	24.6	13.9
Securities, 12 months or more, fair value		7.9
Securities, 12 months or more, gross unrealized losses		0.1
Total, fair value	975.0	826.3
Total, gross unrealized losses	24.6	14.0
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,464.5	276.2
Securities, less than 12 months, gross unrealized losses	39.7	2.4
Securities, 12 months or more, fair value	41.6	108.3
Securities, 12 months or more, gross unrealized losses	2.4	6.4
Total, fair value	1,506.1	384.5
Total, gross unrealized losses	42.1	8.8
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	238.3	208.5
Securities, less than 12 months, gross unrealized losses	4.0	1.3
Total, fair value	238.3	208.5
Total, gross unrealized losses	4.0	1.3
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	727.9	1,149.8
Securities, less than 12 months, gross unrealized losses	18.1	39.0
Securities, 12 months or more, fair value	52.6	70.0
Securities, 12 months or more, gross unrealized losses	1.2	9.0
Total, fair value	780.5	1,219.8
Total, gross unrealized losses	19.3	48.0
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	331.0	479.9
Securities, less than 12 months, gross unrealized losses	6.6	10.8
Securities, 12 months or more, fair value	4.1	12.5
Securities, 12 months or more, gross unrealized losses	0.2	1.9
Total, fair value	335.1	492.4
Total, gross unrealized losses	6.8	12.7
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	652.0	511.1
Securities, less than 12 months, gross unrealized losses	11.4	6.5
Securities, 12 months or more, fair value	43.4	250.6
Securities, 12 months or more, gross unrealized losses	1.1	5.9
Total, fair value	695.4	761.7
Total, gross unrealized losses	12.5	12.4
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	148.9	593.1
Securities, less than 12 months, gross unrealized losses	1.4	9.4
Securities, 12 months or more, fair value	117.7	15.1
Securities, 12 months or more, gross unrealized losses	2.2	0.2
Total, fair value	266.6	608.2
Total, gross unrealized losses	3.6	9.6
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	334.7	1,164.8
Securities, less than 12 months, gross unrealized losses	1.6	27.2
Securities, 12 months or more, fair value	550.4	265.0
Securities, 12 months or more, gross unrealized losses	10.1	11.8
Total, fair value	885.1	1,429.8
Total, gross unrealized losses	$ 11.7	$ 39.0