Amount and Nature of Net Assets Restricted from Payment of Dividends (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Restricted Assets [Line Items]
Total Restricted Net Assets	$ 519,201		$ 462,459
Insurance Companies
Restricted Assets [Line Items]
Total Restricted Net Assets	17,226	[1]	13,985	[1]
Insurance Companies | Fixed maturities
Restricted Assets [Line Items]
Total Restricted Net Assets	11,728		12,489
Insurance Companies | Short-term investments
Restricted Assets [Line Items]
Total Restricted Net Assets	290		290
Insurance Companies | Cash
Restricted Assets [Line Items]
Total Restricted Net Assets	5,208		1,206
Lloyd's Operations
Restricted Assets [Line Items]
Total Restricted Net Assets	501,975	[2]	448,474	[2]
Lloyd's Operations | Fixed maturities
Restricted Assets [Line Items]
Total Restricted Net Assets	428,142		386,608
Lloyd's Operations | Short-term investments
Restricted Assets [Line Items]
Total Restricted Net Assets	72,984		59,928
Lloyd's Operations | Cash
Restricted Assets [Line Items]
Total Restricted Net Assets	$ 849		$ 1,938

[1] The restricted net assets for the Insurance Companies primarily consist of fixed maturities on deposit with various state insurance departments. As of December 31, 2012 restricted net assets in the form of cash include approximately $4.0 million in collateral held in trust in connection with the commercial surety business. The remaining cash as of December 31, 2012 and 2011, as presented in the table above, was on deposit with a U.K. bank to comply with the regulatory requirements of the Financial Services Authority for the underwriting activities of the U.K. Branch.

[2] The restricted net assets for the Lloyd's Operations consists of fixed maturities and cash held in trust for the benefit of syndicate policyholders and short term investments primarily consisting of overseas deposits in various countries with Lloyd's to support underwriting activities in those countries.