Credit Facilities - Additional Information (Detail) In Millions, unless otherwise specified	9 Months Ended		0 Months Ended			0 Months Ended		0 Months Ended
	Sep. 30, 2014	Mar. 28, 2011 USD ($)	Nov. 06, 2014 Syndicate 1221 Subsequent Event	Nov. 06, 2014 Syndicate 1221 Subsequent Event AUD	Nov. 06, 2014 Syndicate 1221 Subsequent Event GBP (£)	Nov. 06, 2014 Syndicate 1221 Subsequent Event Above Floating Rate Line of Credit	Sep. 30, 2014 Lloyd's Syndicate 1221 USD ($)	Nov. 22, 2012 Amended and Restated Credit Agreement	Nov. 22, 2012 Amended and Restated Credit Agreement USD ($)
Line of Credit Facility [Line Items]
Line of credit facility		$ 165							$ 165
Issuance of new letters of credit, expiry date								Dec. 31, 2014
Aggregate face amount of letters of credit outstanding							150.8
Cash collateral posted							1.0
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 September 30, 2014.		As of November 6, 2014 the Company was in compliance with all covenants.
Line of credit facility, maximum borrowing amount				8.0
Line of credit facility, underwriting reserve required					£ 75
Line of credit facility, interest rate						2.00%