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	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 4,187.2	$ 5,491.7
Securities, less than 12 months, gross unrealized losses	57.5	146.6
Securities, 12 months or more, fair value	431.4	809.8
Securities, 12 months or more, gross unrealized losses	11.8	17.2
Total, fair value	4,618.6	6,301.5
Total, gross unrealized losses	69.3	163.8
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	104.1	619.4
Securities, less than 12 months, gross unrealized losses	2.0	39.2
Total, fair value	104.1	619.4
Total, gross unrealized losses	2.0	39.2
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	104.1	619.4
Securities, less than 12 months, gross unrealized losses	2.0	39.2
Total, fair value	104.1	619.4
Total, gross unrealized losses	2.0	39.2
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	4,083.1	4,872.3
Securities, less than 12 months, gross unrealized losses	55.5	107.4
Securities, 12 months or more, fair value	431.4	809.8
Securities, 12 months or more, gross unrealized losses	11.8	17.2
Total, fair value	4,514.5	5,682.1
Total, gross unrealized losses	67.3	124.6
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	749.1	975.0
Securities, less than 12 months, gross unrealized losses	11.1	24.6
Securities, 12 months or more, fair value	80.3
Securities, 12 months or more, gross unrealized losses	3.0
Total, fair value	829.4	975.0
Total, gross unrealized losses	14.1	24.6
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	778.4	1,464.5
Securities, less than 12 months, gross unrealized losses	14.3	39.7
Securities, 12 months or more, fair value	81.1	41.6
Securities, 12 months or more, gross unrealized losses	4.2	2.4
Total, fair value	859.5	1,506.1
Total, gross unrealized losses	18.5	42.1
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	505.3	238.3
Securities, less than 12 months, gross unrealized losses	4.3	4.0
Total, fair value	505.3	238.3
Total, gross unrealized losses	4.3	4.0
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	648.9	727.9
Securities, less than 12 months, gross unrealized losses	11.7	18.1
Securities, 12 months or more, fair value	30.0	52.6
Securities, 12 months or more, gross unrealized losses	0.3	1.2
Total, fair value	678.9	780.5
Total, gross unrealized losses	12.0	19.3
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	371.1	331.0
Securities, less than 12 months, gross unrealized losses	4.4	6.6
Securities, 12 months or more, fair value	0.5	4.1
Securities, 12 months or more, gross unrealized losses		0.2
Total, fair value	371.6	335.1
Total, gross unrealized losses	4.4	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	563.7	652.0
Securities, less than 12 months, gross unrealized losses	8.2	11.4
Securities, 12 months or more, fair value	47.6	43.4
Securities, 12 months or more, gross unrealized losses	1.1	1.1
Total, fair value	611.3	695.4
Total, gross unrealized losses	9.3	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	76.7	148.9
Securities, less than 12 months, gross unrealized losses	0.5	1.4
Securities, 12 months or more, fair value	56.5	117.7
Securities, 12 months or more, gross unrealized losses	2.2	2.2
Total, fair value	133.2	266.6
Total, gross unrealized losses	2.7	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	389.9	334.7
Securities, less than 12 months, gross unrealized losses	1.0	1.6
Securities, 12 months or more, fair value	135.4	550.4
Securities, 12 months or more, gross unrealized losses	1.0	10.1
Total, fair value	525.3	885.1
Total, gross unrealized losses	$ 2.0	$ 11.7