Significant Accounting Policies - Additional Information (Details) - USD ($) shares in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2015	Jan. 01, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Discount rate future net revenues					10.00%
Ceiling test limitation			$ 0	$ 0
Inventory	$ 17,747,000		$ 17,747,000				$ 13,450,000
Contingently issuable shares	2.6	4.2	2.6	4.2
Addition to retained earnings	$ (3,243,968,000)		$ (3,243,968,000)				$ (3,270,605,000)
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Significant Accounting Policies [Line Items]
Addition to retained earnings						$ 1,800,000
Pipe and Production Equipment
Significant Accounting Policies [Line Items]
Inventory	16,400,000		16,400,000
Crude Oil Inventory
Significant Accounting Policies [Line Items]
Inventory	$ 1,300,000		$ 1,300,000
Reserve Fund for Interest Claims
Significant Accounting Policies [Line Items]
Restricted cash		$ 400,000,000		$ 400,000,000
Reserve Fund for Unpaid Professional Fees
Significant Accounting Policies [Line Items]
Restricted cash		$ 35,000,000		$ 35,000,000