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	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 3,760.1	$ 5,491.7
Securities, less than 12 months, gross unrealized losses	41.7	146.6
Securities, 12 months or more, fair value	1,266.3	809.8
Securities, 12 months or more, gross unrealized losses	34.0	17.2
Total, fair value	5,026.4	6,301.5
Total, gross unrealized losses	75.7	163.8
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	998.5	619.4
Securities, less than 12 months, gross unrealized losses	16.2	39.2
Total, fair value	998.5	619.4
Total, gross unrealized losses	16.2	39.2
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	998.5	619.4
Securities, less than 12 months, gross unrealized losses	16.2	39.2
Total, fair value	998.5	619.4
Total, gross unrealized losses	16.2	39.2
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	2,761.6	4,872.3
Securities, less than 12 months, gross unrealized losses	25.5	107.4
Securities, 12 months or more, fair value	1,266.3	809.8
Securities, 12 months or more, gross unrealized losses	34.0	17.2
Total, fair value	4,027.9	5,682.1
Total, gross unrealized losses	59.5	124.6
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	626.0	975.0
Securities, less than 12 months, gross unrealized losses	5.9	24.6
Securities, 12 months or more, fair value	242.9
Securities, 12 months or more, gross unrealized losses	8.1
Total, fair value	868.9	975.0
Total, gross unrealized losses	14.0	24.6
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	404.5	1,464.5
Securities, less than 12 months, gross unrealized losses	2.5	39.7
Securities, 12 months or more, fair value	358.4	41.6
Securities, 12 months or more, gross unrealized losses	12.3	2.4
Total, fair value	762.9	1,506.1
Total, gross unrealized losses	14.8	42.1
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	408.2	238.3
Securities, less than 12 months, gross unrealized losses	4.9	4.0
Securities, 12 months or more, fair value	76.0
Securities, 12 months or more, gross unrealized losses	2.8
Total, fair value	484.2	238.3
Total, gross unrealized losses	7.7	4.0
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	399.3	727.9
Securities, less than 12 months, gross unrealized losses	5.0	18.1
Securities, 12 months or more, fair value	206.3	52.6
Securities, 12 months or more, gross unrealized losses	3.2	1.2
Total, fair value	605.6	780.5
Total, gross unrealized losses	8.2	19.3
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	236.6	331.0
Securities, less than 12 months, gross unrealized losses	1.6	6.6
Securities, 12 months or more, fair value	117.3	4.1
Securities, 12 months or more, gross unrealized losses	1.8	0.2
Total, fair value	353.9	335.1
Total, gross unrealized losses	3.4	6.8
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	429.7	652.0
Securities, less than 12 months, gross unrealized losses	4.7	11.4
Securities, 12 months or more, fair value	132.8	43.4
Securities, 12 months or more, gross unrealized losses	2.7	1.1
Total, fair value	562.5	695.4
Total, gross unrealized losses	7.4	12.5
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	24.8	148.9
Securities, less than 12 months, gross unrealized losses	0.2	1.4
Securities, 12 months or more, fair value	41.8	117.7
Securities, 12 months or more, gross unrealized losses	2.1	2.2
Total, fair value	66.6	266.6
Total, gross unrealized losses	2.3	3.6
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	232.5	334.7
Securities, less than 12 months, gross unrealized losses	0.7	1.6
Securities, 12 months or more, fair value	90.8	550.4
Securities, 12 months or more, gross unrealized losses	1.0	10.1
Total, fair value	323.3	885.1
Total, gross unrealized losses	$ 1.7	$ 11.7