Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			1 Months Ended		3 Months Ended	9 Months Ended			9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Apr. 30, 2013 United Kingdom	Apr. 30, 2012 United Kingdom	Mar. 31, 2012 United Kingdom	Sep. 30, 2013 United Kingdom	Sep. 30, 2013 United Kingdom Syndicate 1221 Minimum	Sep. 30, 2013 United Kingdom Syndicate 1221 Maximum	Sep. 30, 2013 State and Local Jurisdiction Minimum	Sep. 30, 2013 State and Local Jurisdiction Maximum
Income Taxes [Line Items]
Foreign effective income tax rate									35.00%	35.00%
Percentage of premiums											50.00%
Corporate tax rate						23.00%	24.00%	26.00%
Undistributed earnings of non-U.S. subsidiaries	$ 18,900,000		$ 18,900,000
Taxes payable on undistributed earnings	2,000,000		2,000,000
Income tax (benefit) expense	9,804,000	5,225,000	22,731,000	14,731,000
Effective income tax rate	31.50%	28.80%	31.60%	29.20%
Deferred Tax Assets: State and Local	500,000		500,000		500,000
State and local net operating loss carry-forwards	$ 100,000		$ 100,000		$ 200,000
State and local tax carry-forwards expiration year												2023	2031