Derivative Financial Instruments - Commodity Contracts, etc. (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair values of derivative financial instruments
Asset Derivatives	$ 36,209	$ 152,503
Liability Derivatives	(42,890)	(46,990)
Exchange-Traded Derivatives | Broker margin deposits
Fair values of derivative financial instruments
Asset Derivatives	31,645	126,113
Liability Derivatives	(30,192)	(42,496)
Forward Derivative Contracts | Derivative assets
Fair values of derivative financial instruments
Asset Derivatives	4,564	26,390
Forward Derivative Contracts | Derivative liabilities
Fair values of derivative financial instruments
Liability Derivatives	(12,698)	(4,494)
Derivatives designated as hedging instruments
Fair values of derivative financial instruments
Asset Derivatives		5,121
Liability Derivatives	(3,838)
Derivatives designated as hedging instruments | Exchange-Traded Derivatives | Broker margin deposits
Fair values of derivative financial instruments
Asset Derivatives		5,121
Liability Derivatives	(3,838)
Derivatives not designated as hedging instruments
Fair values of derivative financial instruments
Asset Derivatives	36,209	147,382
Liability Derivatives	(39,052)	(46,990)
Derivatives not designated as hedging instruments | Exchange-Traded Derivatives | Broker margin deposits
Fair values of derivative financial instruments
Asset Derivatives	31,645	120,992
Liability Derivatives	(26,354)	(42,496)
Derivatives not designated as hedging instruments | Forward Derivative Contracts | Derivative assets
Fair values of derivative financial instruments
Asset Derivatives	4,564	26,390
Derivatives not designated as hedging instruments | Forward Derivative Contracts | Derivative liabilities
Fair values of derivative financial instruments
Liability Derivatives	$ (12,698)	$ (4,494)