Financial Instruments - Fair value of financial assets and liabilties (Details) - Commodity Derivatives [Member] - Fair Value, Inputs, Level 1 [Member] - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015
Fair Value of Financial Assets and Liabilities
Derivative Asset, Fair Value, Gross Asset		$ 2,991	$ 131
Derivative Asset, Fair Value, Gross Liability	[1]	(2,991)	(131)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		0	0
Derivative Liability, Fair Value, Gross Liability		9,426	470
Derivative Liability, Fair Value, Gross Asset	[1]	(2,991)	(131)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		$ 6,435	$ 339

[1] *Relates primarily to exchange traded futures. Gain and loss positions on multiple contracts are settled net on a daily basis with the exchange.