Derivatives - Offsetting of Derivative Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Offsetting Assets [Line Items]
Gross Amounts, Assets	$ 13,324	$ 542,421
Gross Amounts Offset, Assets	(10,371)	(540,442)
Net Amounts Presented in the Statements of Financial Condition, Assets	2,953	1,979
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets	2,953	1,979
Gross Amounts, Liabilities	12,907	631,571
Gross Amounts Offset, Liabilities	(10,371)	(540,442)
Net Amounts Presented in the Statements of Financial Condition, Liabilities	2,536	91,129
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities	2,536	91,129
Foreign Exchange Swaps [Member]
Offsetting Assets [Line Items]
Gross Amounts, Assets	3,555	1,938
Gross Amounts Offset, Assets	(961)	(327)
Net Amounts Presented in the Statements of Financial Condition, Assets	2,594	1,611
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets	2,594	1,611
Gross Amounts, Liabilities	3,053	2,399
Gross Amounts Offset, Liabilities	(961)	(327)
Net Amounts Presented in the Statements of Financial Condition, Liabilities	2,092	2,072
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities	2,092	2,072
Foreign Exchange/Commodities Options [Member]
Offsetting Assets [Line Items]
Gross Amounts, Assets		482,751
Gross Amounts Offset, Assets		(482,712)
Net Amounts Presented in the Statements of Financial Condition, Assets		39
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets		39
Gross Amounts, Liabilities	42	570,728
Gross Amounts Offset, Liabilities		(482,712)
Net Amounts Presented in the Statements of Financial Condition, Liabilities	42	88,016
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities	42	88,016
Forwards [Member]
Offsetting Assets [Line Items]
Gross Amounts, Assets	33	253
Gross Amounts Offset, Assets	(5)	(17)
Net Amounts Presented in the Statements of Financial Condition, Assets	28	236
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets	28	236
Gross Amounts, Liabilities	176	656
Gross Amounts Offset, Liabilities	(5)	(17)
Net Amounts Presented in the Statements of Financial Condition, Liabilities	171	639
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities	171	639
Futures [Member]
Offsetting Assets [Line Items]
Gross Amounts, Assets	9,736	57,479
Gross Amounts Offset, Assets	(9,405)	(57,386)
Net Amounts Presented in the Statements of Financial Condition, Assets	331	93
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets	331	93
Gross Amounts, Liabilities	9,636	57,788
Gross Amounts Offset, Liabilities	(9,405)	(57,386)
Net Amounts Presented in the Statements of Financial Condition, Liabilities	231	402
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities	$ 231	$ 402