Credit Quality Distribution of Reinsurance Recoverables for Ceded Paid and Unpaid Losses and Loss Adjustment Expenses and Ceded Unearned Premiums Based On Insurer Financial Strength Ratings from A. M. Best or S&P (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Ceded Credit Risk [Line Items]
Rating	AA
Fair Value	$ 1,150,436	[1],[2],[3]
Percent of Total	100.00%	[1],[4]
Superior
Ceded Credit Risk [Line Items]
Rating	A++, A+	[1]
Fair Value	567,903	[1],[2]
Percent of Total	49.00%	[1],[4]
Excellent
Ceded Credit Risk [Line Items]
Rating	A, A-	[1]
Fair Value	568,630	[1],[2]
Percent of Total	50.00%	[1],[4]
Very Good
Ceded Credit Risk [Line Items]
Rating	B++, B+	[1]
Fair Value	67	[1],[2]
Percent of Total	0.00%	[1],[4]
Not rated
Ceded Credit Risk [Line Items]
Rating	NR	[1]
Fair Value	$ 13,836	[1],[2]
Percent of Total	1.00%	[1],[4]

[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.

[4] The Company holds offsetting collateral of approximately 19.7%, including 17.6% for B++ and B+ companies and 54.4% from non rated companies which includes letters of credit, ceded balances payable and other balances held by our Insurance Companies and our Lloyd's Operations.