Debt - Additional Information (Detail) £ in Millions, AUD in Millions				12 Months Ended
	Nov. 07, 2016 USD ($) Tranche	Nov. 04, 2016 GBP (£)	Oct. 04, 2013 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 AUD	Dec. 31, 2016 USD ($)	Nov. 07, 2016 GBP (£) Tranche
Line of Credit Facility [Line Items]
Issuance of new letters of credit, expiry date				Dec. 31, 2018
Line of credit facility, covenant terms				The Bilateral and Club Facilities contain 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 and our subsidiaries, the occurrence of certain material judgments, or a change in control of our Company. As of December 31, 2017, our Company was in compliance with all covenants.
Senior Notes
Line of Credit Facility [Line Items]
Unamortized discount				$ 1,100,000			$ 1,300,000
5.75% Senior Notes
Line of Credit Facility [Line Items]
Interest rate stated on senior notes				5.75%	5.75%	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			$ 265,000,000
Interest rate stated on senior notes			5.75%
Net proceeds from issuance of senior notes			$ 263,300,000
Debt instrument, covenant compliance				As of December 31, 2017, 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%	7.00%	7.00%
Senior notes due date				May 01, 2016
Call premium on redemption of Senior notes				$ 17,900,000
Club Facility
Line of Credit Facility [Line Items]
Number of tranches | Tranche	2							2
Club Facility | Minimum
Line of Credit Facility [Line Items]
Credit facility fee rate	0.95%
Club Facility | Maximum
Line of Credit Facility [Line Items]
Credit facility fee rate	1.60%
Club Facility | Tranche One Extending
Line of Credit Facility [Line Items]
Line of credit, commitment	$ 140,000,000
Club Facility | Tranche Two Extending
Line of Credit Facility [Line Items]
Line of credit, commitment | £								£ 60.0
Bilateral Facility
Line of Credit Facility [Line Items]
Line of credit facility	$ 25,000,000
Additional extended term	2 years
Bilateral Facility | Minimum
Line of Credit Facility [Line Items]
Credit facility fee rate	0.85%
Bilateral Facility | Maximum
Line of Credit Facility [Line Items]
Credit facility fee rate	1.20%
Syndicate 1221
Line of Credit Facility [Line Items]
Aggregate face amount of letters of credit outstanding | AUD						AUD 24.0
Line of credit facility, covenant compliance				As of December 31, 2017, our Company was in compliance with all covenants.
Syndicate 1221 | Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility, interest rate		1.55%
Lloyd's | Syndicate 1221
Line of Credit Facility [Line Items]
Line of credit facility, underwriting reserve required | £		£ 75.0
Lloyd's | Syndicate 1221 | Club Facility
Line of Credit Facility [Line Items]
Aggregate face amount of letters of credit outstanding				$ 125,000,000	£ 60.0
Cash collateral posted				1,100,000
Lloyd's | Syndicate 1221 | Bilateral Facility
Line of Credit Facility [Line Items]
Aggregate face amount of letters of credit outstanding				$ 25,000,000