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	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 1,359.2	$ 363.9
Securities, less than 12 months, gross unrealized losses	46.6	28.8
Securities, 12 months or more, fair value	15.3	45.7
Securities, 12 months or more, gross unrealized losses	1.8	3.6
Total, fair value	1,374.5	409.6
Total, gross unrealized losses	48.4	32.4
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	726.6	88.4
Securities, less than 12 months, gross unrealized losses	6.6	2.6
Securities, 12 months or more, fair value	15.3	45.7
Securities, 12 months or more, gross unrealized losses	1.8	3.6
Total, fair value	741.9	134.1
Total, gross unrealized losses	8.4	6.2
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	23.6
Securities, less than 12 months, gross unrealized losses	0.1
Total, fair value	23.6
Total, gross unrealized losses	0.1
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	375.2
Securities, less than 12 months, gross unrealized losses	1.7
Securities, 12 months or more, fair value	5.4	6.8
Securities, 12 months or more, gross unrealized losses	0.3	0.3
Total, fair value	380.6	6.8
Total, gross unrealized losses	2.0	0.3
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	40.5
Total, fair value	40.5
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	112.5	47.4
Securities, less than 12 months, gross unrealized losses	0.4	1.8
Securities, 12 months or more, fair value		5.1
Securities, 12 months or more, gross unrealized losses		0.3
Total, fair value	112.5	52.5
Total, gross unrealized losses	0.4	2.1
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	68.7	7.6
Securities, less than 12 months, gross unrealized losses	0.4	0.3
Securities, 12 months or more, fair value		6.1
Securities, 12 months or more, gross unrealized losses		0.2
Total, fair value	68.7	13.7
Total, gross unrealized losses	0.4	0.5
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	78.9	4.2
Securities, less than 12 months, gross unrealized losses	3.8	0.1
Securities, 12 months or more, fair value	1.2	18.7
Securities, 12 months or more, gross unrealized losses		1.3
Total, fair value	80.1	22.9
Total, gross unrealized losses	3.8	1.4
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	23.7	7.8
Securities, less than 12 months, gross unrealized losses	0.2	0.3
Securities, 12 months or more, fair value	8.4	8.5
Securities, 12 months or more, gross unrealized losses	1.5	1.5
Total, fair value	32.1	16.3
Total, gross unrealized losses	1.7	1.8
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	3.5	21.4
Securities, less than 12 months, gross unrealized losses		0.1
Securities, 12 months or more, fair value	0.3	0.5
Total, fair value	3.8	21.9
Total, gross unrealized losses		0.1
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	632.6	275.5
Securities, less than 12 months, gross unrealized losses	40.0	26.2
Total, fair value	632.6	275.5
Total, gross unrealized losses	40.0	26.2
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	632.6	275.5
Securities, less than 12 months, gross unrealized losses	40.0	26.2
Total, fair value	632.6	275.5
Total, gross unrealized losses	$ 40.0	$ 26.2