Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Segment	Jun. 30, 2013
Segment Reporting Information [Line Items]
Number of underwriting segments			2
Combined ratio indicating underwriting profit			100.00%
Combined ratio indicating underwriting loss			100.00%
Net earned premiums	$ 231,084	$ 205,814	$ 456,356	$ 408,142
Insurance Companies
Segment Reporting Information [Line Items]
Net earned premiums	178,412	156,547	344,296	310,878
Insurance Companies | UK Branch
Segment Reporting Information [Line Items]
Net earned premiums	$ 10,500	$ 12,700	$ 22,100	$ 22,800