Losses and Loss Adjustment Expenses - Total Losses and Loss Adjustment Expense Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	$ 3,537,995		$ 3,283,862
IBNR	3,033,338		2,584,612
Fair value adjustments	(137,488)		(148,325)
Total	6,433,845	$ 6,641,507	5,720,149	$ 6,143,471	$ 5,724,623	$ 4,509,421
Non-Life Run-Off
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	2,991,786		2,757,774
IBNR	2,385,358		1,991,009
Fair value adjustments	(151,017)		(163,329)
Total	5,226,127	5,459,216	4,585,454	5,064,137	4,693,262	3,435,010
Atrium
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	69,268		68,913
IBNR	129,422		115,613
Fair value adjustments	14,534		16,491
Total	213,224	209,919	201,017	205,499	202,873	212,611
StarStone
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	476,941		457,175
IBNR	518,558		477,990
Fair value adjustments	(1,005)		(1,487)
Total	$ 994,494	$ 972,372	$ 933,678	$ 873,835	$ 828,488	$ 861,800