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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 2,749.1	$ 5,491.7
Securities, less than 12 months, gross unrealized losses	24.5	146.6
Securities, 12 months or more, fair value	1,354.7	809.8
Securities, 12 months or more, gross unrealized losses	32.6	17.2
Total, fair value	4,103.8	6,301.5
Total, gross unrealized losses	57.1	163.8
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	145.7	619.4
Securities, less than 12 months, gross unrealized losses	3.8	39.2
Total, fair value	145.7	619.4
Total, gross unrealized losses	3.8	39.2
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	145.7	619.4
Securities, less than 12 months, gross unrealized losses	3.8	39.2
Total, fair value	145.7	619.4
Total, gross unrealized losses	3.8	39.2
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	2,603.4	4,872.3
Securities, less than 12 months, gross unrealized losses	20.7	107.4
Securities, 12 months or more, fair value	1,354.7	809.8
Securities, 12 months or more, gross unrealized losses	32.6	17.2
Total, fair value	3,958.1	5,682.1
Total, gross unrealized losses	53.3	124.6
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	447.8	975.0
Securities, less than 12 months, gross unrealized losses	4.4	24.6
Securities, 12 months or more, fair value	416.6
Securities, 12 months or more, gross unrealized losses	13.2
Total, fair value	864.4	975.0
Total, gross unrealized losses	17.6	24.6
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	240.0	1,464.5
Securities, less than 12 months, gross unrealized losses	1.5	39.7
Securities, 12 months or more, fair value	267.3	41.6
Securities, 12 months or more, gross unrealized losses	5.1	2.4
Total, fair value	507.3	1,506.1
Total, gross unrealized losses	6.6	42.1
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	321.9	238.3
Securities, less than 12 months, gross unrealized losses	2.7	4.0
Securities, 12 months or more, fair value	72.2
Securities, 12 months or more, gross unrealized losses	2.0
Total, fair value	394.1	238.3
Total, gross unrealized losses	4.7	4.0
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	568.8	727.9
Securities, less than 12 months, gross unrealized losses	6.1	18.1
Securities, 12 months or more, fair value	207.3	52.6
Securities, 12 months or more, gross unrealized losses	3.6	1.2
Total, fair value	776.1	780.5
Total, gross unrealized losses	9.7	19.3
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	417.4	331.0
Securities, less than 12 months, gross unrealized losses	3.0	6.6
Securities, 12 months or more, fair value	159.4	4.1
Securities, 12 months or more, gross unrealized losses	3.5	0.2
Total, fair value	576.8	335.1
Total, gross unrealized losses	6.5	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	284.2	652.0
Securities, less than 12 months, gross unrealized losses	1.6	11.4
Securities, 12 months or more, fair value	131.5	43.4
Securities, 12 months or more, gross unrealized losses	3.0	1.1
Total, fair value	415.7	695.4
Total, gross unrealized losses	4.6	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	112.2	148.9
Securities, less than 12 months, gross unrealized losses	0.5	1.4
Securities, 12 months or more, fair value	34.7	117.7
Securities, 12 months or more, gross unrealized losses	1.8	2.2
Total, fair value	146.9	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	211.1	334.7
Securities, less than 12 months, gross unrealized losses	0.9	1.6
Securities, 12 months or more, fair value	65.7	550.4
Securities, 12 months or more, gross unrealized losses	0.4	10.1
Total, fair value	276.8	885.1
Total, gross unrealized losses	$ 1.3	$ 11.7