Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Income Taxes [Line Items]
Undistributed earnings of non-U.S. subsidiaries	$ 94,400,000		$ 94,400,000
Taxes payable on undistributed earnings	1,700,000		1,700,000
Income tax (benefit) expense	5,225,000	4,476,000	14,731,000	5,015,000
Effective income tax rate	28.80%	24.40%	29.20%	24.40%
Deferred Tax Assets: State and Local	200,000		200,000		200,000
State and local net operating loss carry-forwards	$ 100,000		$ 100,000		$ 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	26.60%	28.60%	26.80%	28.60%
State and Local Jurisdiction | Maximum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year			2032
State and Local Jurisdiction | Minimum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year			2024