Summary of Written Premium (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	Dec. 31, 2013	Dec. 31, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct									$ 1,184,538	$ 1,127,331	$ 1,034,658
Assumed									247,815	243,187	251,807
Ceded									(432,215)	(482,596)	(452,810)
Net Written Premiums	228,007	228,417	231,864	311,850	223,445	196,556	198,469	269,452	1,000,138	887,922	833,655
Direct									1,133,336	1,069,677	972,844
Assumed									243,215	228,247	205,759
Ceded									(440,656)	(455,985)	(396,639)
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
Direct									631,730	552,381	608,945
Assumed									143,681	155,313	151,137
Ceded									(230,182)	(188,733)	(262,649)
Net Losses and LAE	$ 134,658	$ 135,284	$ 140,220	$ 135,067	$ 131,385	$ 125,086	$ 131,148	$ 131,342	$ 545,229	$ 518,961	$ 497,433