Credit Facility - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2011
Line of Credit Facility [Line Items]
Line of credit facility	$ 140
Line of credit facility, expiration date	Mar. 31, 2011
Issuance of new letters of credit, expiry date	Dec. 31, 2011
Line of credit facility, covenant terms		This credit facility contains customary covenants for facilities of this type, including restrictions on indebtedness and liens, limitations on mergers, dividends and the sale of assets, and requirements as to maintaining certain consolidated tangible net worth, statutory surplus and other financial ratios. The credit facility also provides for customary events of default, including failure to pay principal, interest or fees when due, failure to comply with covenants, any representation or warranty made by the Company being false in any material respect, default under certain other indebtedness, certain insolvency or receivership events affecting the Company and its subsidiaries, the occurrence of certain material judgments, or a change in control of the Company.
Line of credit facility, covenant compliance		The Company was in compliance with all covenants under the credit facility as of December 31, 2011.
Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Line of credit facility	165
Line of credit facility, expiration date	Dec. 31, 2012
Lloyd's Operations | Syndicate 1221
Line of Credit Facility [Line Items]
Issuance of new letters of credit, expiry date		Dec. 31, 2011
Aggregate face amount of letters of credit outstanding		149.6