Reinsurance - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Reserve for uncollectible reinsurance amount	$ 13,000,000
Reinsurance rating description	To meet our standards of acceptability, when the reinsurance is placed, a reinsurer generally must have a rating from A.M. Best Company (“A.M. Best”) and/or S&P of “A” or better, or an equivalent financial strength if not rated, plus at least $250 million in policyholders’ surplus.
Reinsurance recoverables	1,053,398,000	[1]
Percentage of reinsurance recoverable of 20 largest reinsurers	74.70%
Charges for uncollectible reinsurance amounts	50,000		(800,000)	2,000,000
Reinsurance recoverables for paid and unpaid losses	889,236,000
Hurricane Gustav and Ike
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Reinsurance recoverables for paid and unpaid losses	50,900,000
Deepwater Horizon
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Reinsurance recoverables for paid and unpaid losses	80,900,000
Minimum
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Policyholders' surplus for reinsurance	$ 500,000,000

[1] Net of reserve for uncollectible reinsurance of approximately $13.0 million.