Net Derivative Gains (Losses) Recognized in Statements of Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	$ (24,228)	$ 3,371
Future | Short
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	(724)	(1,304)
Future | Long
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	255	3,853
Variance swaps
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	(1,629)	(637)
Total Return Swap
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	(4,902)	(6,043)
Options
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	(16,909)	(1,451)
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	(629)	8,692
Credit Default Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gains (losses)	$ 310	$ 261