Net Derivative Gains (Losses) Recognized in Statements of Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments Gain Loss [Line Items]
Net derivative gains (losses)	$ (49,085)	$ (90,238)	$ (37,457)
Future | Short
Derivative Instruments Gain Loss [Line Items]
Net derivative gains (losses)	(1,413)	(4,578)	(2,002)
Future | Long
Derivative Instruments Gain Loss [Line Items]
Net derivative gains (losses)	2,566	(1,411)	(1,029)
Variance swaps
Derivative Instruments Gain Loss [Line Items]
Net derivative gains (losses)	(3,334)	(3,659)	(2,838)
Total Return Swap
Derivative Instruments Gain Loss [Line Items]
Net derivative gains (losses)	(16,235)	(79,142)	(36,481)
Options (puts and calls)
Derivative Instruments Gain Loss [Line Items]
Net derivative gains (losses)	(33,624)	(3,571)	7,650
Interest Rate Swap
Derivative Instruments Gain Loss [Line Items]
Net derivative gains (losses)	206	(566)	(2,222)
Credit Default Swaps
Derivative Instruments Gain Loss [Line Items]
Net derivative gains (losses)	$ 2,749	$ 2,689	$ (535)