Net Derivative Gains (Losses) Recognized in Statements of Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	$ (6,540)	$ (6,665)	$ (30,768)	$ (3,294)
Future | Short
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	(3)	(1,154)	(727)	(2,459)
Future | Long
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	1,497	3,138	1,752	6,991
Variance swaps
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	(711)	(1,033)	(2,340)	(1,670)
Total Return Swap
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	(3,487)	(10,996)	(8,389)	(17,039)
Options
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	(6,879)	(1,784)	(23,788)	(3,235)
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	1,840	5,079	1,211	13,771
Credit Default Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	$ 1,203	$ 85	$ 1,513	$ 347