Fair Value Measurements - Derivative Instrument Assets and Liabilities Measured at Fair Value (Details) - Fair Value on a Recurring Basis - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Quoted prices in active markets for identical assets (Level 1)
Assets
Derivative instruments, at fair value	$ 0	$ 0
Liabilities
Derivative instruments, at fair value	0	132
Significant other observable inputs (Level 2)
Assets
Derivative instruments, at fair value	349,152	458,460
Liabilities
Derivative instruments, at fair value	43,696	22,810
Significant unobservable inputs (Level 3)
Assets
Derivative instruments, at fair value	0	0
Liabilities
Derivative instruments, at fair value	0	0
Fair Value
Assets
Derivative instruments, at fair value	349,152	458,460
Liabilities
Derivative instruments, at fair value	$ 43,696	$ 22,942

Fair value, after offset of derivative asset, of 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, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 8

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=56946850&loc=d3e41678-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-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 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 9

-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226006-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 10

-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

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