Financial Instruments - Fair value of financial assets and liabilties (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Recurring [Member]
Fair Value of Financial Assets and Liabilities
Derivative Asset		$ 4,706		$ 3,685
Derivative Liability		3,619		4,467
Derivative Assets (Liabilities), at Fair Value, Net		1,087		(782)
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value of Financial Assets and Liabilities
Derivative Asset, Fair Value, Gross Asset		4,790		4,658
Derivative Asset, Fair Value, Gross Liability		(84)		(973)
Derivative Liability, Fair Value, Gross Liability		84		973
Derivative Liability, Fair Value, Gross Asset	[1]	(84)		(973)
Derivative Assets (Liabilities), at Fair Value, Net		4,706		3,685
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value of Financial Assets and Liabilities
Derivative Asset, Fair Value, Gross Asset		0		0
Derivative Liability, Fair Value, Gross Liability		0		2,985
Derivative Assets (Liabilities), at Fair Value, Net		0		(2,985)
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value of Financial Assets and Liabilities
Derivative Asset, Fair Value, Gross Asset		0		0
Derivative Liability, Fair Value, Gross Liability		3,619		1,482
Derivative Assets (Liabilities), at Fair Value, Net		(3,619)		(1,482)
Commodity Contract [Member] | Fair Value, Recurring [Member]
Fair Value of Financial Assets and Liabilities
Derivative Asset	[2]	4,706		3,685
Derivative Liability	[2]	0		0
Commodity Contract [Member] | Fair Value, Recurring [Member] | Not Designated as Hedging Instrument [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value of Financial Assets and Liabilities
Derivative Asset, Fair Value, Gross Asset	[2]	4,790		4,658
Derivative Asset, Fair Value, Gross Liability	[1],[2]	(84)		(973)
Derivative Liability, Fair Value, Gross Liability	[2]	84		973
Derivative Liability, Fair Value, Gross Asset	[1],[2]	(84)		(973)
Foreign Exchange Forward [Member] | Fair Value, Recurring [Member]
Fair Value of Financial Assets and Liabilities
Derivative Liability				2,985
Foreign Exchange Forward [Member] | Fair Value, Recurring [Member] | Not Designated as Hedging Instrument [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value of Financial Assets and Liabilities
Derivative Liability, Fair Value, Gross Liability				2,985
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value of Financial Assets and Liabilities
Derivative Assets (Liabilities), at Fair Value, Net		(3,619)	$ (3,676)	(1,482)	$ 154	$ 110	$ (1,228)
Interest Rate Swap [Member] | Fair Value, Recurring [Member]
Fair Value of Financial Assets and Liabilities
Derivative Liability		3,619		1,482
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Not Designated as Hedging Instrument [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value of Financial Assets and Liabilities
Derivative Liability, Fair Value, Gross Liability		$ 3,619		$ 1,482

[1] (1) Commodity derivatives are subject to netting arrangements.

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