Derivatives and Risk Management (Schedule of Derivative Instruments Notional Amount, Weighted Average Contract Prices and Fair Value) (Details) ft³ in Billions	12 Months Ended
	Dec. 31, 2018 USD ($) $ / bbl $ / MMBTU MBbls ft³
Fixed Price Swaps -2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	220.0
Average price per MMBtu and Bbls | $ / MMBTU	2.93
Fair Value	$ 23,000,000
Fixed Price Swaps -2019 [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	346
Average price per MMBtu and Bbls | $ / bbl	68.74
Fair Value	$ 7,000,000
Fixed Price Swaps -2019 [Member] | Not Designated as Hedging Instrument [Member] | Propane [Member]
Derivative [Line Items]
Volume | MBbls	1,689
Average price per MMBtu and Bbls | $ / bbl	33.12
Fair Value	$ 11,000,000
Fixed Price Swaps -2019 [Member] | Not Designated as Hedging Instrument [Member] | Ethane [Member]
Derivative [Line Items]
Volume | MBbls	3,687
Average price per MMBtu and Bbls | $ / bbl	13.90
Fair Value	$ 4,000,000
Two-way Costless-collars - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	53.0
Fair Value	$ 4,000,000
Two-way Costless-collars - 2019 [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	329
Fair Value	$ 6,000,000
Two-way Costless-collars - 2019 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu and Bbls | $ / MMBTU	2.80
Two-way Costless-collars - 2019 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Floor price per MMBtu and Bbls | $ / bbl	65.00
Two-way Costless-collars - 2019 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Average price per MMBtu and Bbls | $ / MMBTU	2.98
Two-way Costless-collars - 2019 Sold Calls [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Average price per MMBtu and Bbls | $ / bbl	72.30
Three-way Costless-collars - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	170.0
Fair Value	$ 8,000,000
Three-way Costless-collars - 2019 Sold Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu and Bbls | $ / MMBTU	2.48
Three-way Costless-collars - 2019 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu and Bbls | $ / MMBTU	2.90
Three-way Costless-collars - 2019 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Average price per MMBtu and Bbls | $ / MMBTU	3.28
Financial protection on production - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	443.0
Fair Value	$ 35,000,000
Financial protection on production - 2019 [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	675
Fair Value	$ 13,000,000
Fixed Price Swaps -2020 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	24.0
Average price per MMBtu and Bbls | $ / MMBTU	2.88
Fair Value	$ 5,000,000
Fixed Price Swaps -2020 [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	366
Average price per MMBtu and Bbls | $ / bbl	65.68
Fair Value	$ 6,000,000
Fixed Price Swaps -2020 [Member] | Not Designated as Hedging Instrument [Member] | Ethane [Member]
Derivative [Line Items]
Volume | MBbls	732
Average price per MMBtu and Bbls | $ / bbl	13.49
Fair Value	$ 1,000,000
Two-way Costless-collars - 2020 [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	366
Fair Value	$ 4,000,000
Two-way Costless-collars - 2020 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Floor price per MMBtu and Bbls | $ / bbl	60.00
Two-way Costless-collars - 2020 Sold Calls [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Average price per MMBtu and Bbls | $ / bbl	69.80
Three-Way Costless Collars - 2020 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	84.0
Three-way Costless-collars - 2020 Sold Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu and Bbls | $ / MMBTU	2.40
Three-way Costless-collars - 2020 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu and Bbls | $ / MMBTU	2.73
Three-way Costless-collars - 2020 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Average price per MMBtu and Bbls | $ / MMBTU	3.03
Financial protection on production - 2020 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	108.0
Fair Value	$ 5,000,000
Financial protection on production - 2020 [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	732
Fair Value	$ 10,000,000
Three-way Costless-collars - 2021 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	37.0
Fair Value	$ (1,000,000)
Three-Way Costless Collars - 2021 Sold Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu and Bbls | $ / MMBTU	2.35
Three-Way Costless Collars - 2021 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu and Bbls | $ / MMBTU	2.60
Three-Way Costless Collars - 2021 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Average price per MMBtu and Bbls | $ / MMBTU	2.93
Basis Swaps - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	107.0
Basis differential per MMBtu	$ (0.29)
Fair Value	$ (10,000,000)
Basis Swaps - 2020 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	59.0
Basis differential per MMBtu	$ (0.44)
Fair Value	$ (1,000,000)
Basis Swaps [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume | ft³	166.0
Fair Value	$ (11,000,000)
Purchased Fixed Price Swaps - 2019 [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	274
Average price per MMBtu and Bbls | $ / bbl	69.10
Fair Value	$ (6,000,000)
Sold Fixed Price Swaps - 2019 [Member] | Not Designated as Hedging Instrument [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	620
Average price per MMBtu and Bbls | $ / bbl	68.90
Fair Value	$ 13,000,000
Purchased Call Options - 2020 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	68.0
Cap price per MMBtu and Bbls | $ / MMBTU	3.63
Fair Value	$ 4,000,000
Purchased Call Options - 2021 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	57.0
Cap price per MMBtu and Bbls | $ / MMBTU	3.52
Fair Value	$ 2,000,000
Purchased Call Options [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	125.0
Fair Value	$ 6,000,000
Sold Call Options - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	52.0
Cap price per MMBtu and Bbls | $ / MMBTU	3.50
Fair Value	$ (3,000,000)
Sold Call Options - 2019 [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	270
Cap price per MMBtu and Bbls | $ / bbl	65.00
Sold Call Options - 2020 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	137.0
Cap price per MMBtu and Bbls | $ / MMBTU	3.39
Fair Value	$ (12,000,000)
Sold Call Options - 2021 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	114.0
Cap price per MMBtu and Bbls | $ / MMBTU	3.33
Fair Value	$ (7,000,000)
Sold Call Options [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	303.0
Fair Value	$ (22,000,000)
Storage Fixed Price Swaps - 2019 [Member]
Derivative [Line Items]
Volume | ft³	0.8
Average price per MMBtu and Bbls | $ / MMBTU	3.03
Storage Basis Swaps - 2019 [Member]
Derivative [Line Items]
Volume | ft³	0.8
Basis differential per MMBtu	$ (0.44)