Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Parenthetical) (Detail) (USD $)	Sep. 30, 2013
Ceded Credit Risk [Line Items]
Reserve for uncollectible reinsurance amount	$ 11,300,000
Collateral Held	240,576,000	[1]
Ceded Balances Payable
Ceded Credit Risk [Line Items]
Collateral Held	180,600,000
Letter of Credit
Ceded Credit Risk [Line Items]
Collateral Held	57,200,000
All Other
Ceded Credit Risk [Line Items]
Collateral Held	$ 2,800,000

[1] Collateral of $240.6 million consists of $180.6 million in ceded balances payable, $57.2 million in letters of credit, and $2.8 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.