Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Income Taxes [Line Items]
Undistributed earnings of non-U.S. subsidiaries	$ 18,000,000
Taxes payable on undistributed earnings	1,900,000
Income tax (benefit) expense	6,643,000	3,281,000
Effective income tax rate	32.30%	29.30%
Deferred Tax Assets: State and Local	400,000		500,000
State and local net operating loss carry-forwards	$ 200,000		$ 200,000
United Kingdom
Income Taxes [Line Items]
Foreign effective income tax rate	35.00%
Corporate tax rate	24.00%	26.00%
United Kingdom | Syndicate 1221 | Maximum
Income Taxes [Line Items]
Percentage of premiums	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	23.00%
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