Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Parenthetical) (Detail) (USD $)	Jun. 30, 2013
Ceded Credit Risk [Line Items]
Reserve for uncollectible reinsurance amount	$ 10,800,000
Collateral Held	243,872,000	[1]
Ceded Balances Payable
Ceded Credit Risk [Line Items]
Collateral Held	183,700,000
Letter of Credit
Ceded Credit Risk [Line Items]
Collateral Held	57,200,000
All Other
Ceded Credit Risk [Line Items]
Collateral Held	$ 3,000,000

[1] Collateral of $243.9 million consists of $183.7 million in ceded balances payable, $57.2 million in letters of credit, and $3.0 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.