Debt - Additional Information (Detail) $ in Thousands, £ in Millions					12 Months Ended
	Nov. 20, 2015 USD ($)	Nov. 24, 2014 USD ($)	Nov. 06, 2014 AUD	Oct. 04, 2013 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2014 USD ($)	Nov. 06, 2014 GBP (£)
Line of Credit Facility [Line Items]
Issuance of new letters of credit, expiry date	Dec. 31, 2016
Line of credit facility, covenant terms					Each of the November 20, 2015 and November 24, 2014 credit facilities 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. These credit facilities also provide 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 our Company being false in any material respect, default under certain other indebtedness, certain insolvency or receivership events affecting our Company, the occurrence of certain material judgments, or a change in control of our Company
Interest rate stated on senior notes					7.00%
Senior notes due date					May 01, 2016
Call premium on redemption of Senior notes						$ 17,895
Senior Notes
Line of Credit Facility [Line Items]
Unamortized discount					$ 1,400		$ 1,600
Minimum
Line of Credit Facility [Line Items]
Credit facility fee rate	0.85%	0.95%
Maximum
Line of Credit Facility [Line Items]
Credit facility fee rate	1.20%	1.60%
Syndicate 1221
Line of Credit Facility [Line Items]
Aggregate face amount of letters of credit outstanding					$ 8,000
Line of credit facility, covenant compliance					As of December 31, 2015, our Company was in compliance with all covenants.
Syndicate 1221 | Above Floating Rate | Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility, interest rate			2.00%
Lloyd's | Syndicate 1221
Line of Credit Facility [Line Items]
Line of credit facility, underwriting reserve required | £								£ 75.0
Aggregate face amount of letters of credit outstanding	$ 20,000	$ 175,000
Cash collateral posted					$ 3,300
Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility	$ 25,000	$ 175,000
Credit Facility | Syndicate 1221
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing amount | AUD			AUD 8,000,000
5.75% Senior Notes
Line of Credit Facility [Line Items]
Interest rate stated on senior notes					5.75%
Senior notes due date					Oct. 15, 2023
5.75% Senior Notes | Senior Notes
Line of Credit Facility [Line Items]
Principal amount of senior notes				$ 265,000
Interest rate stated on senior notes				5.75%
Net proceeds from issuance of senior notes				$ 263,000
Debt instrument, covenant compliance					As of December 31, 2015, our Company was in compliance with all such covenants
7.0% Senior Notes | Senior Notes
Line of Credit Facility [Line Items]
Interest rate stated on senior notes					7.00%
Senior notes due date					May 01, 2016
Call premium on redemption of Senior notes					$ 17,900