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, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 6,096.0	$ 2,771.7
Securities, less than 12 months, gross unrealized losses	193.0	117.2
Securities, 12 months or more, fair value	121.2	9.8
Securities, 12 months or more, gross unrealized losses	7.4	1.3
Total, fair value	6,217.2	2,781.5
Total, gross unrealized losses	200.4	118.5
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	318.9	796.1
Securities, less than 12 months, gross unrealized losses	19.5	102.8
Total, fair value	318.9	796.1
Total, gross unrealized losses	19.5	102.8
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	318.9	796.1
Securities, less than 12 months, gross unrealized losses	19.5	102.8
Total, fair value	318.9	796.1
Total, gross unrealized losses	19.5	102.8
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	5,777.1	1,975.6
Securities, less than 12 months, gross unrealized losses	173.5	14.4
Securities, 12 months or more, fair value	121.2	9.8
Securities, 12 months or more, gross unrealized losses	7.4	1.3
Total, fair value	5,898.3	1,985.4
Total, gross unrealized losses	180.9	15.7
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	386.6	7.1
Securities, less than 12 months, gross unrealized losses	4.8	0.1
Securities, 12 months or more, fair value	4.9
Securities, 12 months or more, gross unrealized losses	0.4
Total, fair value	391.5	7.1
Total, gross unrealized losses	5.2	0.1
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	2,832.2	1,006.6
Securities, less than 12 months, gross unrealized losses	97.9	4.4
Securities, 12 months or more, fair value	81.2	0.5
Securities, 12 months or more, gross unrealized losses	4.0
Total, fair value	2,913.4	1,007.1
Total, gross unrealized losses	101.9	4.4
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	412.6	102.3
Securities, less than 12 months, gross unrealized losses	6.1	0.5
Securities, 12 months or more, fair value	6.6
Securities, 12 months or more, gross unrealized losses	0.2
Total, fair value	419.2	102.3
Total, gross unrealized losses	6.3	0.5
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	552.0	404.4
Securities, less than 12 months, gross unrealized losses	19.9	2.1
Securities, 12 months or more, fair value	10.9
Securities, 12 months or more, gross unrealized losses	0.3
Total, fair value	562.9	404.4
Total, gross unrealized losses	20.2	2.1
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	303.2	111.4
Securities, less than 12 months, gross unrealized losses	4.0	0.3
Securities, 12 months or more, fair value	1.6
Total, fair value	304.8	111.4
Total, gross unrealized losses	4.0	0.3
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	756.5	291.4
Securities, less than 12 months, gross unrealized losses	30.6	5.7
Securities, 12 months or more, fair value	11.1	0.6
Securities, 12 months or more, gross unrealized losses	2.4
Total, fair value	767.6	292.0
Total, gross unrealized losses	33.0	5.7
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	320.6	50.2
Securities, less than 12 months, gross unrealized losses	8.9	1.3
Securities, 12 months or more, fair value	4.5	7.8
Securities, 12 months or more, gross unrealized losses	0.1	1.3
Total, fair value	325.1	58.0
Total, gross unrealized losses	9.0	2.6
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	213.4	2.2
Securities, less than 12 months, gross unrealized losses	1.3
Securities, 12 months or more, fair value	0.4	0.9
Total, fair value	213.8	3.1
Total, gross unrealized losses	$ 1.3