Reinsurance - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Entity		Dec. 31, 2011	Dec. 31, 2010
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Reserve for uncollectible reinsurance amount	$ 11,500,000		$ 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 $500 million in policyholders’ surplus.
Reinsurance recoverables	1,150,436,000	[1],[2],[3]
Ceded premiums earned	396,639,000		346,172,000	338,647,000
Ceded losses incurred	262,600,000		213,400,000	281,300,000
Percentage of reinsurance recoverable of 20 largest reinsurers	72.70%
Number of largest reinsurers	20
Percentage of funding requirement to retain	100.00%
Reinsurance recoverables for paid and unpaid losses	929,421,000
Letter of Credit
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Percentage of collateral held	25.00%
Catastrophic
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Reinsurance recoverables for paid and unpaid losses	88,200,000
Superstorm Sandy
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Reinsurance recoverables for paid and unpaid losses	54,600,000
Hurricane Gustav and Ike
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Reinsurance recoverables for paid and unpaid losses	33,600,000		50,900,000
Marine Business
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Reinsurance recoverables for paid and unpaid losses	22,900,000
Deepwater Horizon
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Reinsurance recoverables for paid and unpaid losses	64,000,000		80,900,000
Minimum
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Policyholders' surplus for reinsurance	$ 500,000,000
Percentage of collateral required of qualifying reinsurers	0.00%
Maximum
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Percentage of collateral required of qualifying reinsurers	100.00%

[1] Equivalent S&P rating used for certain companies when an A.M. Best rating was unavailable.

[2] Net of reserve for uncollectible reinsurance of approximately $11.5 million. The carrying value consists of reinsurance recoverables on paid losses due within 30-45 days and reinsurance on unpaid losses which by nature of our reserving process is our best estimate of the value as of December 31, 2012.

[3] Net of reserve for uncollectible reinsurance of approximately $11.5 million.