Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Taxes [Line Items]
Undistributed earnings of non-U.S. subsidiaries	$ 17,900,000								$ 17,900,000
Taxes payable on undistributed earnings	1,800,000								1,800,000
Income tax (benefit) expense	13,243,000	5,225,000	6,225,000	3,281,000	2,122,000	4,476,000	5,032,000	(4,493,000)	27,974,000	7,137,000	29,251,000
Effective income tax rate									30.50%	21.80%	29.60%
Deferred Tax Assets: State and Local	500,000				200,000				500,000	200,000
State and local net operating loss carry-forwards	$ 200,000				$ 200,000				$ 200,000	$ 200,000
United Kingdom
Income Taxes [Line Items]
Foreign effective income tax rate									35.00%
Corporate tax rate									26.00%
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%
United Kingdom | Expected Future Tax Rate
Income Taxes [Line Items]
Corporate tax rate									24.00%
Net investment income
Income Taxes [Line Items]
Effective income tax rate									26.80%	27.60%	27.00%
State and Local Jurisdiction | Maximum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year									2031
State and Local Jurisdiction | Minimum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year									2023