Losses and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at beginning of period	$ 6,641,507	$ 5,724,623	$ 5,720,149	$ 4,509,421
Less: reinsurance reserves recoverable			1,383,474
Less: deferred charges on retroactive insurance	254,300	0	255,911	0
Net balance as at June 30	5,084,469	4,206,521	4,103,856	3,355,225
Net incurred losses and LAE:
Current period	126,634	121,335	241,936	234,349
Prior periods	(30,172)	(55,435)	(62,256)	(98,313)
Total net incurred losses and LAE	96,462	65,900	179,680	136,036
Net paid losses:
Current period	(17,022)	(21,490)	(22,356)	(32,654)
Prior periods	(203,010)	(194,485)	(389,413)	(312,641)
Net losses paid	(220,032)	(215,975)	(411,769)	(345,295)
Effect of exchange rate movement	(28,127)	24,723	(23,246)	(29,423)
Acquired on purchase of subsidiaries	10,019	0	10,019	774,758
Assumed business	0	305,763	1,084,251	495,631
Net balance as at June 30	4,942,791	4,386,932	4,942,791	4,386,932
Plus: reinsurance reserves recoverable	1,265,020		1,265,020
Plus: deferred charge on retroactive reinsurance	247,272	265,426	247,272	265,426
Balance as at June 30	6,433,845	6,143,471	6,433,845	6,143,471
Non-Life Run-off Segment, Atrium Segment, and Starstone Segment
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: reinsurance reserves recoverable	1,302,738	1,518,102	1,360,382	1,154,196
Net paid losses:
Plus: reinsurance reserves recoverable	1,243,782	1,491,113	1,243,782	1,491,113
Non-Life Run-Off
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at beginning of period	5,459,216	4,693,262	4,585,454	3,435,010
Less: reinsurance reserves recoverable	977,096	1,210,933	1,034,747	800,709
Less: deferred charges on retroactive insurance	254,300	0	255,911	0
Net balance as at June 30	4,227,820	3,482,329	3,294,796	2,634,301
Net incurred losses and LAE:
Current period	518	22,547	6,587	43,273
Prior periods	(25,208)	(51,417)	(54,831)	(85,243)
Total net incurred losses and LAE	(24,690)	(28,870)	(48,244)	(41,970)
Net paid losses:
Current period	(2,058)	(9,434)	(4,048)	(14,005)
Prior periods	(140,998)	(155,006)	(271,321)	(215,695)
Net losses paid	(143,056)	(164,440)	(275,369)	(229,700)
Effect of exchange rate movement	(18,963)	25,876	(14,323)	(12,362)
Acquired on purchase of subsidiaries	10,019	0	10,019	774,758
Assumed business	0	305,763	1,084,251	495,631
Net balance as at June 30	4,051,130	3,620,658	4,051,130	3,620,658
Plus: reinsurance reserves recoverable	927,725	1,178,053	927,725	1,178,053
Plus: deferred charge on retroactive reinsurance	247,272	265,426	247,272	265,426
Balance as at June 30	5,226,127	5,064,137	5,226,127	5,064,137
Atrium
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at beginning of period	209,919	202,873	201,017	212,611
Less: reinsurance reserves recoverable	26,249	26,629	25,852	28,278
Net balance as at June 30	183,670	176,244	175,165	184,333
Net incurred losses and LAE:
Current period	20,568	17,495	36,631	32,373
Prior periods	(3,435)	(3,738)	(3,909)	(11,596)
Total net incurred losses and LAE	17,133	13,757	32,722	20,777
Net paid losses:
Current period	(5,255)	(4,538)	(7,493)	(7,408)
Prior periods	(7,268)	(7,583)	(12,778)	(16,624)
Net losses paid	(12,523)	(12,121)	(20,271)	(24,032)
Effect of exchange rate movement	(1,912)	1,608	(1,248)	(1,590)
Net balance as at June 30	186,368	179,488	186,368	179,488
Plus: reinsurance reserves recoverable	26,856	26,011	26,856	26,011
Balance as at June 30	213,224	205,499	213,224	205,499
StarStone
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at beginning of period	972,372	828,488	933,678	861,800
Less: reinsurance reserves recoverable	299,393	280,540	299,783	325,209
Net balance as at June 30	672,979	547,948	633,895	536,591
Net incurred losses and LAE:
Current period	105,548	81,293	198,718	158,703
Prior periods	(1,529)	(280)	(3,516)	(1,474)
Total net incurred losses and LAE	104,019	81,013	195,202	157,229
Net paid losses:
Current period	(9,709)	(7,518)	(10,815)	(11,241)
Prior periods	(54,744)	(31,896)	(105,314)	(80,322)
Net losses paid	(64,453)	(39,414)	(116,129)	(91,563)
Effect of exchange rate movement	(7,252)	(2,761)	(7,675)	(15,471)
Net balance as at June 30	705,293	586,786	705,293	586,786
Plus: reinsurance reserves recoverable	289,201	287,049	289,201	287,049
Balance as at June 30	$ 994,494	$ 873,835	$ 994,494	$ 873,835