Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Income Taxes [Line Items]
Undistributed earnings of non-U.S. subsidiaries	$ 72,300,000
Taxes payable on undistributed earnings	1,500,000
Income tax (benefit) expense	3,281,000	(4,493,000)
Effective income tax rate	29.30%	36.30%
Deferred Tax Assets: State and Local	500,000		200,000
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	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	24.00%
Net investment income
Income Taxes [Line Items]
Effective income tax rate	25.20%	28.50%
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	2024