Losses and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at January 1				$ 4,509,421					$ 4,509,421
Net incurred losses and LAE:
Current period									476,364	$ 327,817	$ 93,442
Prior periods									(372,031)	(318,671)	(257,114)
Total net incurred losses and LAE	$ (64,062)	$ 32,359	$ 65,900	70,136	$ (56,085)	$ 17,533	$ 59,749	$ (12,051)	104,333	9,146	(163,672)
Net paid losses:
Plus: deferred charge on retroactive reinsurance	255,911				0				255,911	0	0
Balance as at December 31	5,720,149				4,509,421				5,720,149	4,509,421
Non-Life Run-of Segment, Atrium Segment, and Starstone Segment
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at January 1				4,509,421				4,219,905	4,509,421	4,219,905	3,650,127
Less: reinsurance reserves recoverable				1,154,196				1,146,588	1,154,196	1,146,588	876,220
Net balance as at January 1									3,355,225	3,073,317	2,773,907
Net incurred losses and LAE:
Current period									476,364	327,817	93,442
Prior periods									(372,031)	(318,671)	(257,114)
Total net incurred losses and LAE									104,333	9,146	(163,672)
Net paid losses:
Current period									(99,933)	(166,796)	(41,282)
Prior periods									(681,956)	(420,715)	(360,214)
Total net paid losses									(781,889)	(587,511)	(401,496)
Effect of exchange rate movement									(65,069)	(69,804)	6,222
Acquired on purchase of subsidiaries									878,815	901,447	757,850
Assumed business									612,441	28,630	100,506
Net balance as at December 31	4,103,856				3,355,225				4,103,856	3,355,225	3,073,317
Plus: reinsurance reserves recoverable	1,360,382				1,154,196				1,360,382	1,154,196	1,146,588
Plus: deferred charge on retroactive reinsurance	255,911				0				255,911	0	0
Balance as at December 31	5,720,149				4,509,421				5,720,149	4,509,421	4,219,905
Non-Life Run-Off Segment
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at January 1				3,435,010				4,004,513	3,435,010	4,004,513	3,650,127
Less: reinsurance reserves recoverable				800,709				1,121,533	800,709	1,121,533	876,220
Net balance as at January 1									2,634,301	2,882,980	2,773,907
Net incurred losses and LAE:
Current period									39,924	24,235	74,139
Prior periods									(310,754)	(288,946)	(257,114)
Total net incurred losses and LAE									(270,830)	(264,711)	(182,975)
Net paid losses:
Current period									(16,049)	(87,681)	(10,656)
Prior periods									(501,246)	(312,415)	(360,214)
Total net paid losses									(517,295)	(400,096)	(370,870)
Effect of exchange rate movement									(42,636)	(49,267)	4,936
Acquired on purchase of subsidiaries									878,815	436,765	557,476
Assumed business									612,441	28,630	100,506
Net balance as at December 31	3,294,796				2,634,301				3,294,796	2,634,301	2,882,980
Plus: reinsurance reserves recoverable	1,034,747				800,709				1,034,747	800,709	1,121,533
Plus: deferred charge on retroactive reinsurance	255,911				0				255,911	0	0
Balance as at December 31	4,585,454				3,435,010				4,585,454	3,435,010	4,004,513
Atrium
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at January 1				212,611				215,392	212,611	215,392	0
Less: reinsurance reserves recoverable				28,278				25,055	28,278	25,055	0
Net balance as at January 1									184,333	190,337	0
Net incurred losses and LAE:
Current period									69,400	74,094	19,303
Prior periods									(21,921)	(18,666)	0
Total net incurred losses and LAE									47,479	55,428	19,303
Net paid losses:
Current period									(21,145)	(29,626)	(30,626)
Prior periods									(30,890)	(27,985)	0
Total net paid losses									(52,035)	(57,611)	(30,626)
Effect of exchange rate movement									(4,612)	(3,821)	1,286
Acquired on purchase of subsidiaries									0	0	200,374
Net balance as at December 31	175,165				184,333				175,165	184,333	190,337
Plus: reinsurance reserves recoverable	25,852				28,278				25,852	28,278	25,055
Balance as at December 31	201,017				212,611				201,017	212,611	215,392
StarStone Segment
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at January 1				861,800				0	861,800	0
Less: reinsurance reserves recoverable				$ 325,209				$ 0	325,209	0
Net balance as at January 1									536,591	0
Net incurred losses and LAE:
Current period									367,040	229,488
Prior periods									(39,356)	(11,059)
Total net incurred losses and LAE									327,684	218,429
Net paid losses:
Current period									(62,739)	(49,489)
Prior periods									(149,820)	(80,315)
Total net paid losses									(212,559)	(129,804)
Effect of exchange rate movement									(17,821)	(16,716)
Acquired on purchase of subsidiaries									0	464,682
Assumed business									0	0
Net balance as at December 31	633,895				536,591				633,895	536,591
Plus: reinsurance reserves recoverable	299,783				325,209				299,783	325,209	0
Balance as at December 31	$ 933,678				$ 861,800				$ 933,678	$ 861,800	$ 0