Derivative Instruments - Gain From Derivative Instruments (Details) - Not designated as hedging instruments - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
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	$ 65,292	$ 144,787	$ 273,653	$ 580,760
Futures
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	35,097	197,650	249,274	618,245
Currency forwards
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	8,950	(52,796)	3,135	(37,076)
Options
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	21,120	(65)	21,119	(411)
Interest rate swaps
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	$ 125	$ (2)	$ 125	$ 2