Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Income Taxes [Line Items]
Undistributed earnings of non-U.S. subsidiaries	$ 21,200,000		$ 21,200,000
Taxes payable on undistributed earnings	2,200,000		2,200,000
Unrecognized tax benefits	0	0	0	0
Interest or penalties related to unrecognized tax benefits	0	0	0	0
Income tax (benefit) expense	15,032,000	9,804,000	36,384,000	22,731,000
Effective income tax rate	32.50%	31.50%	32.40%	31.60%
Deferred Tax Assets: State and Local	400,000		400,000		600,000
State and local net operating loss carry-forwards	$ 100,000		$ 100,000		$ 100,000
United Kingdom
Income Taxes [Line Items]
Foreign effective income tax rate			35.00%
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 States
Income Taxes [Line Items]
U.S. tax rate			35.00%
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