Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes [Line Items]
Undistributed earnings of non-U.S. subsidiaries	$ 21,500,000								$ 21,500,000
Taxes payable on undistributed earnings	2,500,000								2,500,000
Unrecognized tax benefits	0				$ 0				0	$ 0	$ 0
Interest or penalties related to unrecognized tax benefits									0	0	0
Income tax (benefit) expense	6,072,000	$ 8,723,000	$ 9,195,000	$ 12,427,000	8,823,000	$ 15,032,000	$ 7,998,000	$ 13,354,000	$ 36,417,000	$ 45,207,000	$ 28,807,000
Effective income tax rate									31.00%	32.20%	31.20%
Deferred Tax Assets: State and Local	$ 721,000				$ 777,000				$ 721,000	$ 777,000
United Kingdom
Income Taxes [Line Items]
Foreign effective income tax rate									35.00%
United Kingdom | Syndicate 1221 | Maximum
Income Taxes [Line Items]
Percentage of premiums	50.00%								50.00%
United Kingdom | Syndicate 1221 | Minimum
Income Taxes [Line Items]
Foreign effective income tax rate									35.00%
State and Local Jurisdiction | Maximum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year										2033
State and Local Jurisdiction | Minimum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year										2025