Acquisitions (Details) $ / shares in Units, shares in Millions				2 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended			15 Months Ended
		Dec. 21, 2016 USD ($) $ / shares shares	Oct. 04, 2016 USD ($)	Feb. 21, 2019 a	Sep. 30, 2018 USD ($) a	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($) a $ / shares	Dec. 31, 2017 USD ($) a $ / shares	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) $ / shares
Business Acquisition [Line Items]
Payments to acquire oil and gas property and equipment								$ 33,255,000	$ 89,896,000	$ 2,183,790,000
Carrying value of properties exchanged or acquired								95,121,000		$ 733,000
Wells in progress								295,529,000	321,347,000		$ 321,347,000
Proved oil and gas properties								7,278,362,000	6,139,379,000		6,139,379,000
Unproved oil and gas properties								$ 1,581,401,000	$ 2,047,203,000		$ 2,047,203,000
Common Stock, par value per share | $ / shares		$ 0.01						$ 0.01	$ 0.01		$ 0.01
Rock Oil Acquisition 2016 [Member]
Business Acquisition [Line Items]
Payments to acquire a business			$ 991,000,000						$ 7,700,000		$ 998,691,000
Wells in progress			5,672,000
Proved oil and gas properties			82,584,000
Unproved oil and gas properties			913,819,000
Other assets			5,338,000
Total fair value of oil and gas properties acquired			1,007,413,000
Working capital			(1,127,000)
Asset retirement obligations			(7,595,000)
Total fair value of net assets acquired			$ 998,691,000
Other Martin County Acquisition 2018 [Member]
Business Acquisition [Line Items]
Net acres acquired | a								1,030
Payments to acquire oil and gas property and equipment								$ 33,300,000
Other Howard and Martin Counties acquisitions 2017 [Member]
Business Acquisition [Line Items]
Net acres acquired | a									3,600
Payments to acquire oil and gas property and equipment									$ 76,500,000
QStar Acquisition 2016 [Member]
Business Acquisition [Line Items]
Payments to acquire oil and gas property and equipment		$ 1,200,000,000				$ 7,300,000	$ 1,174,628,000
Carrying value of properties exchanged or acquired		1,611,822,000
Wells in progress		21,812,000
Proved oil and gas properties		61,239,000
Unproved oil and gas properties		1,538,264,000
Working capital		(1,852,000)
Asset retirement obligations		$ (7,641,000)
Private issuance of Common Stock for an acquisition | shares		13.4
Total consideration							$ 1,611,822,000
Fair value of equity consideration	[1]	$ 437,194,000
Total oil and gas properties acquired		$ 1,621,315,000
Subsequent Event [Member]
Business Acquisition [Line Items]
Non-monetary trade, acreage acquired | a				1,580
Non-monetary trade, acreage exchanged | a				1,650
Howard and Martin Counties Trades 2018 [Member]
Business Acquisition [Line Items]
Non-monetary trade, acreage acquired | a					2,650
Carrying value of properties exchanged or acquired					$ 95,100,000
Gain (loss) recognized on trade					$ 0
Other Howard and Martin Counties trades [Member]
Business Acquisition [Line Items]
Non-monetary trade, acreage acquired | a									8,125
Carrying value of properties exchanged or acquired									$ 293,963,000
Non-monetary trade, acreage exchanged | a									7,580

[1] (1) The Company issued approximately 13.4 million shares of common stock, par value $0.01 per share, in a private placement to the sellers in the QStar Acquisition on December 21, 2016. The equity consideration was valued on this date using Level 1 and Level 2 inputs with a discount applied due to the lack of marketability in the near term in accordance with the Lock-Up and Registration Rights Agreement that prohibited the sale of such stock until no earlier than the 90th day after issuance.