Gross Unrealized Investment Losses and Related Fair Values for Debt Securities and Equity Securities Grouped by Length of Time in Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 2,654.0	$ 2,771.7
Securities, less than 12 months, gross unrealized losses	74.5	117.2
Securities, 12 months or more, fair value	160.6	9.8
Securities, 12 months or more, gross unrealized losses	4.4	1.3
Total, fair value	2,814.6	2,781.5
Total, gross unrealized losses	78.9	118.5
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	602.5	796.1
Securities, less than 12 months, gross unrealized losses	56.3	102.8
Total, fair value	602.5	796.1
Total, gross unrealized losses	56.3	102.8
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	602.5	796.1
Securities, less than 12 months, gross unrealized losses	56.3	102.8
Total, fair value	602.5	796.1
Total, gross unrealized losses	56.3	102.8
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	2,051.5	1,975.6
Securities, less than 12 months, gross unrealized losses	18.2	14.4
Securities, 12 months or more, fair value	160.6	9.8
Securities, 12 months or more, gross unrealized losses	4.4	1.3
Total, fair value	2,212.1	1,985.4
Total, gross unrealized losses	22.6	15.7
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	65.4	7.1
Securities, less than 12 months, gross unrealized losses	0.2	0.1
Total, fair value	65.4	7.1
Total, gross unrealized losses	0.2	0.1
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	807.7	1,006.6
Securities, less than 12 months, gross unrealized losses	6.3	4.4
Securities, 12 months or more, fair value	136.8	0.5
Securities, 12 months or more, gross unrealized losses	2.8
Total, fair value	944.5	1,007.1
Total, gross unrealized losses	9.1	4.4
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	62.6	102.3
Securities, less than 12 months, gross unrealized losses	0.5	0.5
Total, fair value	62.6	102.3
Total, gross unrealized losses	0.5	0.5
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	228.5	404.4
Securities, less than 12 months, gross unrealized losses	2.0	2.1
Securities, 12 months or more, fair value	5.4
Total, fair value	233.9	404.4
Total, gross unrealized losses	2.0	2.1
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	85.4	111.4
Securities, less than 12 months, gross unrealized losses	0.7	0.3
Securities, 12 months or more, fair value	4.7
Total, fair value	90.1	111.4
Total, gross unrealized losses	0.7	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	620.8	291.4
Securities, less than 12 months, gross unrealized losses	7.5	5.7
Securities, 12 months or more, fair value	8.8	0.6
Securities, 12 months or more, gross unrealized losses	1.3
Total, fair value	629.6	292.0
Total, gross unrealized losses	8.8	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	133.1	50.2
Securities, less than 12 months, gross unrealized losses	0.9	1.3
Securities, 12 months or more, fair value	4.4	7.8
Securities, 12 months or more, gross unrealized losses	0.3	1.3
Total, fair value	137.5	58.0
Total, gross unrealized losses	1.2	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	48.0	2.2
Securities, less than 12 months, gross unrealized losses	0.1
Securities, 12 months or more, fair value	0.5	0.9
Total, fair value	48.5	3.1
Total, gross unrealized losses	$ 0.1