Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses	$ 93,749	$ 104,165
(Increase) reduction in provisions for bad debt	1,448	19,814
Reduction in estimates of unallocated loss adjustment expense liabilities	(6,693)	(13,381)
Amortization of fair value adjustments	(2,390)	(834)
Reductions in IBNR reserves	(86)	(15,349)
Net (reduction) increase in ultimate losses and loss adjustment expense liabilities	83,218	70,136
Reduction in estimates of unallocated loss adjustment expense liabilities		14,000
Non-Life Run-Off Segment
Effects of Reinsurance [Line Items]
Losses and loss adjustment expenses relating to prior periods	5,600	19,300
Increase (reduction) in estimates of net ultimate losses	(13,535)	20,982
(Increase) reduction in provisions for bad debt	1,448	19,814
Reduction in estimates of unallocated loss adjustment expense liabilities	(7,788)	(13,975)
Amortization of fair value adjustments	(783)	(293)
Reductions in IBNR reserves	(37,063)	(37,278)
Net (reduction) increase in ultimate losses and loss adjustment expense liabilities	(23,554)	(13,100)
Reduction in Estimates of Net Ultimate Losses
Effects of Reinsurance [Line Items]
Net Increase (decrease) in ultimate loss and loss adjustment expense liabilities	(23,600)	(13,100)
Losses and loss adjustment expenses relating to prior periods	29,600	33,800
Increase (reduction) in estimates of net ultimate losses	19,200	300
(Increase) reduction in provisions for bad debt	1,400	19,800
Reduction in estimates of unallocated loss adjustment expense liabilities	8,200
Amortization of fair value adjustments	800	300
Reduction in Estimates of Net Ultimate Losses | StarStone Segment
Effects of Reinsurance [Line Items]
Net (reduction) increase in ultimate losses and loss adjustment expense liabilities		$ 20,700
Sussex Insurance Company | Reduction in Estimates of Net Ultimate Losses
Effects of Reinsurance [Line Items]
Net Increase (decrease) in ultimate loss and loss adjustment expense liabilities	$ (6,100)