Derivative Financial Instruments - Summary of Open Commodity Derivative Contracts (Parenthetical) (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Unamortized premium	$ 225,085,000	$ 228,096,000
Commodity Derivative Contract Deferred Premium Put Options \| Natural Gas
Debt Instrument [Line Items]
Unamortized premium	$ 140

Amount, after accumulated amortization, of debt premium.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

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