Derivative Instruments - Gain From Derivative Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Not designated as hedging instruments
Gain impact of derivative instruments not designated as hedging instruments
Gain impact that derivative instruments not designated as hedging instruments had on results of operations	$ 286,046	$ 561,125	$ 1,162,272
Derivative instruments designed as hedging instruments:
Gain impact of derivative instruments not designated as hedging instruments
Net gain from derivative instruments, which are recorded in accumulated other comprehensive income	(642)
Futures | Not designated as hedging instruments
Gain impact of derivative instruments not designated as hedging instruments
Gain impact that derivative instruments not designated as hedging instruments had on results of operations	290,609	559,626	1,180,483
Currency forwards | Not designated as hedging instruments
Gain impact of derivative instruments not designated as hedging instruments
Gain impact that derivative instruments not designated as hedging instruments had on results of operations	2,603	1,915	(16,431)
Options | Not designated as hedging instruments
Gain impact of derivative instruments not designated as hedging instruments
Gain impact that derivative instruments not designated as hedging instruments had on results of operations	$ (7,166)	(410)	(1,784)
Others | Not designated as hedging instruments
Gain impact of derivative instruments not designated as hedging instruments
Gain impact that derivative instruments not designated as hedging instruments had on results of operations		$ (6)	$ 4