Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Segment	Sep. 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	$ 245,367	$ 213,895	$ 701,723	$ 622,037
Insurance Companies
Segment Reporting Information [Line Items]
Net earned premiums	182,051	162,962	526,347	473,840
Insurance Companies | UK Branch
Segment Reporting Information [Line Items]
Net earned premiums	$ 8,900	$ 13,900	$ 31,000	$ 36,700