Derivative Commodity Instruments - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Derivative [Line Items]
Gain on fair value of derivatives	$ 5,455	$ 34,036
Commodity contracts \| Morgan Stanley Capital Group Inc
Derivative [Line Items]
Gain on fair value of derivatives	11,300
Commodity contracts \| BP Corporation North America, Inc
Derivative [Line Items]
Gain on fair value of derivatives	$ 6,100

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5618551-113959

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration