Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 Segment	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of underwriting segments									2
Number of corporate segment									1
Combined ratio indicating underwriting profit									100.00%
Combined ratio indicating underwriting loss									100.00%
Net earned premiums	$ 234,172	$ 245,367	$ 231,084	$ 225,272	$ 219,902	$ 213,895	$ 205,814	$ 202,328	$ 935,895	$ 841,939	$ 781,964
Insurance Companies
Segment Reporting Information [Line Items]
Net earned premiums									704,574	639,338	571,439
Insurance Companies | UK Branch
Segment Reporting Information [Line Items]
Net earned premiums									$ 37,700	$ 44,600	$ 63,900