Reinsurance Balances Recoverable - Summary of Reinsurance Reserves Recoverable (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Effects of Reinsurance [Line Items]
Outstanding losses	$ 702,038		$ 798,798
IBNR	581,333		605,830
Fair value adjustments	(18,351)		(21,154)
Total reinsurance reserves recoverable	1,265,020		1,383,474
Paid losses recoverable	80,095		90,530
Net	1,345,115		1,474,004
Non-Life Run-Off
Effects of Reinsurance [Line Items]
Outstanding losses	502,667		587,164
IBNR	441,240		465,211
Fair value adjustments	(16,182)		(17,628)
Total reinsurance reserves recoverable	927,725	$ 977,096	1,034,747	$ 1,178,053	$ 1,210,933	$ 800,709
Paid losses recoverable	62,454		72,213
Net	990,179		1,106,960
Atrium
Effects of Reinsurance [Line Items]
Outstanding losses	7,142		6,772
IBNR	17,798		16,581
Fair value adjustments	1,916		2,499
Total reinsurance reserves recoverable	26,856	26,249	25,852	26,011	26,629	28,278
Paid losses recoverable	762		430
Net	27,618		26,282
StarStone
Effects of Reinsurance [Line Items]
Outstanding losses	171,289		182,076
IBNR	121,997		123,732
Fair value adjustments	(4,085)		(6,025)
Total reinsurance reserves recoverable	289,201	$ 299,393	299,783	$ 287,049	$ 280,540	$ 325,209
Paid losses recoverable	15,885		16,568
Net	305,086		316,351
Life and Annuities
Effects of Reinsurance [Line Items]
Outstanding losses	20,940		22,786
IBNR	298		306
Fair value adjustments	0		0
Total reinsurance reserves recoverable	21,238		23,092
Paid losses recoverable	994		1,319
Net	$ 22,232		$ 24,411