Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Parenthetical) (Detail) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Ceded Credit Risk [Line Items]
Reserve for uncollectible reinsurance amount	$ 11,500,000		$ 13,000,000
Collateral Held	227,165,000	[1]
Ceded Balances Payable
Ceded Credit Risk [Line Items]
Collateral Held	165,800,000
Letter of Credit
Ceded Credit Risk [Line Items]
Collateral Held	57,200,000
All Other
Ceded Credit Risk [Line Items]
Collateral Held	$ 4,200,000

[1] Collateral of $227.2 million consists of $165.8 million in ceded balances payable, $57.2 million in letters of credit, and $4.2 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.