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, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 5,418.2	$ 3,009.8
Securities, less than 12 months, gross unrealized losses	122.7	119.9
Securities, 12 months or more, fair value	828.8	1,393.5
Securities, 12 months or more, gross unrealized losses	26.0	25.8
Total, fair value	6,247.0	4,403.3
Total, gross unrealized losses	148.7	145.7
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	705.4	514.4
Securities, less than 12 months, gross unrealized losses	39.9	80.8
Total, fair value	705.4	514.4
Total, gross unrealized losses	39.9	80.8
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	705.4	514.4
Securities, less than 12 months, gross unrealized losses	39.9	80.8
Total, fair value	705.4	514.4
Total, gross unrealized losses	39.9	80.8
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	4,712.8	2,495.4
Securities, less than 12 months, gross unrealized losses	82.8	39.1
Securities, 12 months or more, fair value	828.8	1,393.5
Securities, 12 months or more, gross unrealized losses	26.0	25.8
Total, fair value	5,541.6	3,888.9
Total, gross unrealized losses	108.8	64.9
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	366.3	270.5
Securities, less than 12 months, gross unrealized losses	11.5	3.1
Securities, 12 months or more, fair value	8.2	16.3
Securities, 12 months or more, gross unrealized losses	0.1	0.4
Total, fair value	374.5	286.8
Total, gross unrealized losses	11.6	3.5
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,317.1	105.2
Securities, less than 12 months, gross unrealized losses	22.9	0.8
Securities, 12 months or more, fair value	97.7	372.0
Securities, 12 months or more, gross unrealized losses	5.2	8.3
Total, fair value	1,414.8	477.2
Total, gross unrealized losses	28.1	9.1
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	218.2	6.1
Securities, less than 12 months, gross unrealized losses	2.0
Securities, 12 months or more, fair value		5.7
Total, fair value	218.2	11.8
Total, gross unrealized losses	2.0
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	842.4	574.7
Securities, less than 12 months, gross unrealized losses	19.5	17.2
Securities, 12 months or more, fair value	87.5	150.7
Securities, 12 months or more, gross unrealized losses	7.0	4.0
Total, fair value	929.9	725.4
Total, gross unrealized losses	26.5	21.2
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	397.2	133.4
Securities, less than 12 months, gross unrealized losses	10.4	4.1
Securities, 12 months or more, fair value	11.4	26.2
Securities, 12 months or more, gross unrealized losses	0.4	0.4
Total, fair value	408.6	159.6
Total, gross unrealized losses	10.8	4.5
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	609.4	187.9
Securities, less than 12 months, gross unrealized losses	8.5	0.5
Securities, 12 months or more, fair value	388.8	586.4
Securities, 12 months or more, gross unrealized losses	10.8	11.2
Total, fair value	998.2	774.3
Total, gross unrealized losses	19.3	11.7
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	321.7	176.5
Securities, less than 12 months, gross unrealized losses	4.9	0.7
Securities, 12 months or more, fair value	36.7	60.9
Securities, 12 months or more, gross unrealized losses	0.4	0.7
Total, fair value	358.4	237.4
Total, gross unrealized losses	5.3	1.4
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	640.5	1,041.1
Securities, less than 12 months, gross unrealized losses	3.1	12.7
Securities, 12 months or more, fair value	198.5	175.3
Securities, 12 months or more, gross unrealized losses	2.1	0.8
Total, fair value	839.0	1,216.4
Total, gross unrealized losses	$ 5.2	$ 13.5