Credit Facility (Details)		6 Months Ended
	Nov. 01, 2017	Jun. 30, 2018 USD ($)	May 04, 2018 USD ($)	Dec. 31, 2017 USD ($)	Apr. 25, 2017 USD ($)	Oct. 31, 2016 USD ($)
Line Of Credit Facility [Line Items]
Credit facility		$ 421,000,000		$ 388,000,000
Senior Line of Credit | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		$ 1,000,000,000
Borrowing base			$ 600,000,000		$ 550,000,000.0	$ 500,000,000
Interest payable, term	5 years	90 days
Weighted average interest rate (percent)		4.60%		4.06%
Borrowing base threshold (percent)		50.00%
Credit facility		$ 421,000,000		$ 388,000,000
Unused portion of current borrowing base		$ 179,000,000		$ 162,000,000
Senior Line of Credit | Revolving Credit Facility | Borrowing Base Utilization Percentage Less Than 50%
Line Of Credit Facility [Line Items]
Commitment fee payable rate (percent)		0.375%
Senior Line of Credit | Revolving Credit Facility | Borrowing Base Utilization Percentage Equal to or Greater Than 50%
Line Of Credit Facility [Line Items]
Commitment fee payable rate (percent)		0.50%
Senior Line of Credit | Revolving Credit Facility | Minimum
Line Of Credit Facility [Line Items]
Interest payable, term		90 days
Current ratio		1
Senior Line of Credit | Revolving Credit Facility | Minimum | LIBOR Plus Margin Rate
Line Of Credit Facility [Line Items]
Interest rate (percent)	2.00%
Senior Line of Credit | Revolving Credit Facility | Minimum | Prime Rate Plus Margin Rate
Line Of Credit Facility [Line Items]
Interest rate (percent)	1.00%
Senior Line of Credit | Revolving Credit Facility | Maximum
Line Of Credit Facility [Line Items]
Ratio of total debt to EBITDAX		3.5
Senior Line of Credit | Revolving Credit Facility | Maximum | LIBOR Plus Margin Rate
Line Of Credit Facility [Line Items]
Interest rate (percent)	3.00%
Senior Line of Credit | Revolving Credit Facility | Maximum | Prime Rate Plus Margin Rate
Line Of Credit Facility [Line Items]
Interest rate (percent)	2.00%