Derivative Financial Instruments - Summary of Outstanding Oil and Gas Derivative Contracts and Weighted Average Oil and Gas Prices (Details) - Forecast $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($) MMBTU $ / MBbls $ / MMBTU MBbls	Dec. 31, 2019 USD ($) $ / MBbls MBbls	Dec. 31, 2018 USD ($) MMBTU $ / MBbls $ / MMBTU MBbls	Dec. 31, 2017 USD ($) $ / MBbls $ / MMBTU MBbls
Natural Gas [Member] | Purchased | Put Option
Derivative [Line Items]
Notional (MMbtu) | MMBTU	2,850		2,400
Weighted average strike price (in dollars per unit) | $ / MMBTU	2.75		3.25
Natural Gas [Member] | Sold | Put Option
Derivative [Line Items]
Notional (MMbtu) | MMBTU	2,850		2,400
Weighted average strike price (in dollars per unit) | $ / MMBTU	2.36		2.60
Natural Gas [Member] | Sold | Call Option
Derivative [Line Items]
Notional (MMbtu) | MMBTU	2,850		2,400
Weighted average strike price (in dollars per unit) | $ / MMBTU	4.02		4.70
Crude Oil [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume	92	0	183
Price differential ($/Bbl) | $ / MBbls	(55.00)	(0.00)	(55.00)	(55.00)
Crude Oil [Member] | Basis Swap Contracts
Derivative [Line Items]
Price differential ($/Bbl) | $ / MBbls	(1.00)	(0.00)	(0.91)	(1.00)
Crude Oil [Member] | Put Option | Subject to Master Netting Agreement with Counter Party
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume		4,278	4,278	4,278
Fair value of notional MBbls excluded | $	$ 22.4	$ 22.4	$ 22.4	$ 22.4
Crude Oil [Member] | Midland-Cushing Index | Basis Swap Contracts
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume	3,072	0	1,638
Crude Oil [Member] | Purchased | Put Option
Derivative [Line Items]
Weighted average strike price (in dollars per unit) | $ / MBbls	51.08	50.00	51.17
Derivative, Nonmonetary Notional Amount, Volume	7,464	1,200	9,600
Crude Oil [Member] | Sold | Put Option
Derivative [Line Items]
Weighted average strike price (in dollars per unit) | $ / MBbls	41.31	40.00	40.78
Derivative, Nonmonetary Notional Amount, Volume	7,464	1,200	9,600
Natural Gas [Member] | Swap [Member]
Derivative [Line Items]
Price differential ($/Bbl) | $ / MMBTU	(3.43)		(3.50)	(3.43)
Notional (MMbtu) | MMBTU	920		450
Three Way Collars | Crude Oil [Member] | Purchased | Put Option
Derivative [Line Items]
Weighted average strike price (in dollars per unit) | $ / MBbls	0.00	50.00	50.00
Derivative, Nonmonetary Notional Amount, Volume	0	3,000	8,700
Three Way Collars | Crude Oil [Member] | Sold | Put Option
Derivative [Line Items]
Weighted average strike price (in dollars per unit) | $ / MBbls	0.00	40.00	40.00
Derivative, Nonmonetary Notional Amount, Volume	0	3,000	8,700
Three Way Collars | Crude Oil [Member] | Sold | Call Option
Derivative [Line Items]
Weighted average strike price (in dollars per unit) | $ / MBbls	0.00	80.40	75.40
Derivative, Nonmonetary Notional Amount, Volume	0	3,000	8,700
Collars [Member] | Crude Oil [Member] | Purchased | Put Option
Derivative [Line Items]
Weighted average strike price (in dollars per unit) | $ / MBbls	46.75	0.00	45.67
Derivative, Nonmonetary Notional Amount, Volume	736	0	1,095
Collars [Member] | Crude Oil [Member] | Sold | Put Option
Derivative [Line Items]
Weighted average strike price (in dollars per unit) | $ / MBbls	59.85	0.00	61.31
Derivative, Nonmonetary Notional Amount, Volume	736	0	1,095