Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Parenthetical) (Detail) (USD $)	Mar. 31, 2013
Ceded Credit Risk [Line Items]
Reserve for uncollectible reinsurance amount	$ 11,500,000
Collateral Held	225,905,000	[1]
Ceded Balances Payable
Ceded Credit Risk [Line Items]
Collateral Held	165,100,000
Letter of Credit
Ceded Credit Risk [Line Items]
Collateral Held	57,200,000
All Other
Ceded Credit Risk [Line Items]
Collateral Held	$ 3,600,000

[1] Collateral of $225.9 million consists of $165.1 million in ceded balances payable, $57.2 million in letters of credit, and $3.6 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.