Losses and Loss Adjustment Expenses - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Effects of Reinsurance [Line Items]
Reduction in estimates of net ultimate losses	$ (5,020,000)	$ 23,472,000
Reduction in estimates of unallocated loss adjustment expense liabilities	13,307,000	16,404,000
Amortization of fair value adjustments	(3,764,000)	2,093,000
Reserves with an estimate aggregate value	8,300,000
Reductions in IBNR reserves	33,879,000	2,702,000
SeaBright [Member]
Effects of Reinsurance [Line Items]
Net Increase (decrease) in ultimate loss and loss adjustment expense liabilities	13,500,000
Impact of losses incurred	1,400,000
Reserves with an estimate aggregate value	6,800,000
Net ultimate losses on claims settlements	(6,700,000)
Net incurred loss development	26,700,000
SeaBright [Member] | IBNR Reserves [Member]
Effects of Reinsurance [Line Items]
Reductions in IBNR reserves	23,500,000
Reduction in Estimates of Net Ultimate Losses [Member]
Effects of Reinsurance [Line Items]
Net Increase (decrease) in ultimate loss and loss adjustment expense liabilities	29,200,000	9,200,000
Losses and loss adjustment expenses relating to prior periods	30,600,000	19,400,000
Reduction in estimates of net ultimate losses	13,500,000	5,100,000
Reduction in estimates of unallocated loss adjustment expense liabilities	1,330,000
Amortization of fair value adjustments	3,800,000
Reduction in Estimates of Net Ultimate Losses [Member] | SeaBright [Member]
Effects of Reinsurance [Line Items]
Impact of losses incurred	$ 1,400,000	$ 28,500,000