Losses and Loss Adjustment Expenses - Additional Information (Detail) $ in Thousands		12 Months Ended
	Jan. 01, 2014 commutation	Dec. 31, 2015 USD ($) commutation	Dec. 31, 2014 USD ($) commutation	Dec. 31, 2013 USD ($) commutation	Dec. 31, 2012 USD ($)
Effects of Reinsurance [Line Items]
Reduction in estimates of net ultimate losses		$ 179,038	$ 80,660	$ (121,989)
(Increase) reduction in provisions for bad debt		25,271	7,700	(1,999)
Reduction in estimates of unallocated loss adjustment expense liabilities		(59,199)	(53,970)	(49,629)
Amortization of fair value adjustments		9,765	(9,844)	5,947
Reductions in IBNR reserves		$ (324,026)	$ (231,357)	$ (232,931)
Total number of commutations | commutation		79	98	108
Number of partial commutations of the Company's top 10 ceded recoverables | commutation	1		2
Number of commutations of the Company's top 10 assumed exposures | commutation			2
Net change in case and LAE reserves		$ 366,300	$ 285,800	$ 310,500
Net paid losses		501,200	312,400
Net incurred liabilities settled by way of commutation		56,600	29,100	29,800
Assumed incurred liabilities		140,300	99,500	97,300
Ceded incurred reinsurance recoverables		83,700	70,400	67,500
Net (reduction) increase in ultimate losses and loss adjustment expense liabilities		104,333	9,146	(163,672)
Reduction in estimates of unallocated loss adjustment expense liabilities			49,500	49,600
Net change in case and LAE reserves		(278,825)	(275,494)	(290,554)
Estimate of net ultimate liability for asbestos and environmental claims		356,600	389,100
Estimate of gross ultimate liability for asbestos and environmental claims		390,800	439,500
Non-Life Run-Off Segment
Effects of Reinsurance [Line Items]
Losses and loss adjustment expenses relating to prior periods		43,300	31,200
Reduction in estimates of net ultimate losses		(202,814)	(212,102)	(141,341)
(Increase) reduction in provisions for bad debt		25,271	7,700	(1,999)
Reduction in estimates of unallocated loss adjustment expense liabilities		(62,653)	(48,891)	(49,580)
Amortization of fair value adjustments		19,908	3,982	5,947
Reductions in IBNR reserves		(364,774)	(301,784)	(231,278)
Net paid losses				360,200
Net (reduction) increase in ultimate losses and loss adjustment expense liabilities		(270,830)	(264,711)	(182,975)
Net change in case and LAE reserves		(355,335)	(310,414)	(280,933)
Reduction in Estimates of Net Ultimate Losses
Effects of Reinsurance [Line Items]
Net Increase (decrease) in ultimate loss and loss adjustment expense liabilities		(270,800)	(264,700)	(183,000)
Losses and loss adjustment expenses relating to prior periods		310,800	288,900	257,100
Reduction in estimates of net ultimate losses		242,700	235,800	215,500
(Increase) reduction in provisions for bad debt		25,300	7,700
Reduction in estimates of unallocated loss adjustment expense liabilities		62,700
Amortization of fair value adjustments		19,900	4,000
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			24,200
IBNR Reserves
Effects of Reinsurance [Line Items]
Net incurred loss development		135,000	26,600	49,700
Reductions in IBNR reserves		377,700	262,400	265,200	$ 267,400
Decrease in aggregate estimate of IBNR loss reserves related to asbestos liabilities		32,000	59,400	69,800	36,400
Decrease in aggregate estimate of IBNR loss reserves related to environmental liabilities		1,600	6,200	4,900	2,600
Decrease in aggregate estimate of IBNR loss reserves related to general casualty liabilities		3,000	62,500	42,600	96,300
Decrease in aggregate estimate of IBNR loss reserves related to workers compensation liabilities		243,400	63,600	42,100	52,700
Decrease in aggregate estimate of IBNR loss reserves related to other remaining liabilities		97,700	$ 70,700	105,800	$ 79,400
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		$ (39,900)
SeaBright | Reduction in Estimates of Net Ultimate Losses
Effects of Reinsurance [Line Items]
Net (reduction) increase in ultimate losses and loss adjustment expense liabilities				$ 74,100