Derivative Instruments and Hedging Activities - Effect of Derivatives on Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Effect of derivative instruments on the Consolidated Balance Sheet
Derivative Assets	$ 57,665	$ 2,239
Derivative Liabilities	0	30,637
Derivatives Not Designated as Hedges \| Commodity contracts \| Derivative instruments (non-current)
Effect of derivative instruments on the Consolidated Balance Sheet
Derivative Assets	57,665	0
Derivative Liabilities	0	30,637
Derivatives Not Designated as Hedges \| Commodity contracts \| Derivative instruments (non-current)
Effect of derivative instruments on the Consolidated Balance Sheet
Derivative Assets	0	2,239
Derivative Liabilities	$ 0	$ 0

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant