Derivatives - Offsetting of Derivative Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Offsetting Assets [Line Items]
Gross Amounts, Assets	$ 339,760	$ 119,466
Gross Amounts Offset, Assets	(245,016)	(106,797)
Net Amounts Presented in the Statements of Financial Condition, Assets	94,744	12,669
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets	94,744	12,669
Gross Amounts, Liabilities	308,812	114,730
Gross Amounts Offset, Liabilities	(245,016)	(106,797)
Net Amounts Presented in the Statements of Financial Condition, Liabilities	63,796	7,933
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities	63,796	7,933
Foreign Exchange Swaps [Member]
Offsetting Assets [Line Items]
Gross Amounts, Assets	3,586	1,135
Gross Amounts Offset, Assets	(630)	(334)
Net Amounts Presented in the Statements of Financial Condition, Assets	2,956	801
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets	2,956	801
Gross Amounts, Liabilities	2,930	3,180
Gross Amounts Offset, Liabilities	(630)	(334)
Net Amounts Presented in the Statements of Financial Condition, Liabilities	2,300	2,846
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities	2,300	2,846
Foreign Exchange/Commodities Options [Member]
Offsetting Assets [Line Items]
Gross Amounts, Assets	210,086	31,834
Gross Amounts Offset, Assets	(210,085)	(27,418)
Net Amounts Presented in the Statements of Financial Condition, Assets	1	4,416
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets	1	4,416
Gross Amounts, Liabilities	258,918	27,548
Gross Amounts Offset, Liabilities	(210,085)	(27,418)
Net Amounts Presented in the Statements of Financial Condition, Liabilities	48,833	130
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities	48,833	130
Interest Rate Swaps [Member]
Offsetting Assets [Line Items]
Gross Amounts, Assets		242
Gross Amounts Offset, Assets		0
Net Amounts Presented in the Statements of Financial Condition, Assets		242
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets		242
Equity Options [Member]
Offsetting Assets [Line Items]
Gross Amounts, Liabilities		18
Gross Amounts Offset, Liabilities		0
Net Amounts Presented in the Statements of Financial Condition, Liabilities		18
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities		18
Forwards [Member]
Offsetting Assets [Line Items]
Gross Amounts, Assets	85,464	4,363
Gross Amounts Offset, Assets		(101)
Net Amounts Presented in the Statements of Financial Condition, Assets	85,464	4,262
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets	85,464	4,262
Gross Amounts, Liabilities	574	1,584
Gross Amounts Offset, Liabilities		(101)
Net Amounts Presented in the Statements of Financial Condition, Liabilities	574	1,483
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities	574	1,483
Rate Lock Commitments [Member]
Offsetting Assets [Line Items]
Gross Amounts, Assets	6,219	2,923
Gross Amounts Offset, Assets		0
Net Amounts Presented in the Statements of Financial Condition, Assets	6,219	2,923
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets	6,219	2,923
Gross Amounts, Liabilities	12,089	2,390
Gross Amounts Offset, Liabilities		0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	12,089	2,390
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities	12,089	2,390
Futures [Member]
Offsetting Assets [Line Items]
Gross Amounts, Assets	34,405	78,969
Gross Amounts Offset, Assets	(34,301)	(78,944)
Net Amounts Presented in the Statements of Financial Condition, Assets	104	25
Gross Amounts Not Offset, Financial Instruments, Assets		0
Gross Amounts Not Offset, Cash Collateral Received, Assets		0
Net Amount, Assets	104	25
Gross Amounts, Liabilities	34,301	80,010
Gross Amounts Offset, Liabilities	$ (34,301)	(78,944)
Net Amounts Presented in the Statements of Financial Condition, Liabilities		1,066
Gross Amounts Not Offset, Financial Instruments, Liabilities		0
Gross Amounts Not Offset, Cash Collateral Received, Liabilities		0
Net Amount, Liabilities		$ 1,066