Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Income Taxes [Line Items]
Undistributed earnings of non-U.S. subsidiaries	$ 82,200,000		$ 82,200,000
Taxes payable on undistributed earnings	1,600,000		1,600,000
Income tax (benefit) expense	6,225,000	5,032,000	9,506,000	539,000
Effective income tax rate	29.50%	34.60%	29.40%	25.10%
Deferred Tax Assets: State and Local	600,000		600,000		200,000
State and local net operating loss carry-forwards	$ 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	28.20%	28.70%	26.90%	28.60%
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