Derivative Instruments - Gain From Derivative Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Gain impact that derivative instruments not designated as hedging instruments had on results of operations	$ (141,449)	$ 286,046	$ 561,125
Futures | Not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Gain impact that derivative instruments not designated as hedging instruments had on results of operations	(309,598)	290,609	559,626
Currency forwards | Not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Gain impact that derivative instruments not designated as hedging instruments had on results of operations	174,310	2,603	1,915
Options | Not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Gain impact that derivative instruments not designated as hedging instruments had on results of operations	(6,161)	(7,166)	(410)
Others | Not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Gain impact that derivative instruments not designated as hedging instruments had on results of operations	0	0	(6)
Foreign exchange - forward contract | Derivative instruments designed as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Net gain from derivative instruments, which are recorded in accumulated other comprehensive income	$ 63	$ (642)	$ 0