Derivative Instruments and Fair Value Measurements - Narrative (Details) - Commodity Contract	9 Months Ended	12 Months Ended
	Sep. 30, 2018 barrel_of_oil_per_day contract	Dec. 31, 2017 barrel_of_oil_per_day contract
Derivative Instruments, Gain (Loss) [Line Items]
Number of contracts held | contract	12	20
Reported Value Measurement
Derivative Instruments, Gain (Loss) [Line Items]
Number of contracts held | contract	4	4
October through December 2018
Derivative Instruments, Gain (Loss) [Line Items]
Production	258	258
January through April 2019
Derivative Instruments, Gain (Loss) [Line Items]
Production	322
May 2019
Derivative Instruments, Gain (Loss) [Line Items]
Production	258
June 2019 through August 2019
Derivative Instruments, Gain (Loss) [Line Items]
Production	322
January 2018
Derivative Instruments, Gain (Loss) [Line Items]
Production		452
February through May 2018
Derivative Instruments, Gain (Loss) [Line Items]
Production		322
June 2018
Derivative Instruments, Gain (Loss) [Line Items]
Production		258
July 2018
Derivative Instruments, Gain (Loss) [Line Items]
Production		646
August through September 2018
Derivative Instruments, Gain (Loss) [Line Items]
Production		322