Derivatives and Risk Management (Schedule of Derivative Instruments, Notional Amount in BCF, Weighted Average Contract Prices and Fair Value) (Details) - Not Designated as Hedging Instrument [Member] $ in Millions, ft³ in Billions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / bbl $ / MMBTU MBbls ft³
Fixed Price Swaps - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	215
Average price | $ / MMBTU	2.97
Fair Value | $	$ 33
Fixed Price Swaps - 2018 [Member] | Propane [Member]
Derivative [Line Items]
Volume | MBbls	1,100
Average price | $ / bbl	34.64
Fair Value | $	$ 3
Fixed Price Swaps - 2018 [Member] | Ethane [Member]
Derivative [Line Items]
Volume | MBbls	413
Average price | $ / bbl	11.19
Two-Way Costless Collars - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	6
Fair Value | $	$ 1
Two-Way Costless Collars - 2018 Purchased Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.90
Two-Way Costless Collars - 2018 Sold Calls [Member] | Natural Gas [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	3.27
Three-Way Costless Collars - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	213
Fair Value | $	$ 39
Three-Way Costless Collars - 2018 Sold Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.40
Three-Way Costless Collars - 2018 Purchased Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.97
Three-Way Costless Collars - 2018 Sold Calls [Member] | Natural Gas [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	3.37
Financial protection on production - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	434
Fair Value | $	$ 73
Fixed Price Swaps - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	93
Average price | $ / MMBTU	3.00
Fair Value | $	$ 19
Fixed Price Swaps - 2019 [Member] | Ethane [Member]
Derivative [Line Items]
Volume | MBbls	91
Average price | $ / bbl	11.61
Two-Way Costless Collars - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	53
Fair Value | $	$ 6
Two-Way Costless Collars - 2019 Purchased Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.80
Two-Way Costless Collars - 2019 Sold Calls [Member] | Natural Gas [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	2.98
Three-Way Costless Collars - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	133
Fair Value | $	$ 11
Three-Way Costless Collars - 2019 Sold Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.49
Three-Way Costless Collars - 2019 Purchased Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.93
Three-Way Costless Collars - 2019 Sold Calls [Member] | Natural Gas [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	3.34
Financial protection on production - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	279
Fair Value | $	$ 36
Basis Swaps - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	73
Basis differential per MMBtu | $ / MMBTU	(0.59)
Basis Swaps - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	6
Basis differential per MMBtu | $ / MMBTU	1.01
Fair Value | $	$ (1)
Basis Swaps [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	79
Fair Value | $	$ (1)
Purchased Call Options - 2020 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	68
Cap price per MMBtu | $ / MMBTU	3.63
Fair Value | $	$ 7
Purchased Call Options - 2021 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	57
Cap price per MMBtu | $ / MMBTU	3.52
Fair Value | $	$ 11
Purchased Call Options [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	125
Fair Value | $	$ 18
Sold Call Options - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	47
Cap price per MMBtu | $ / MMBTU	3.50
Fair Value | $	$ (1)
Sold Call Options - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	52
Cap price per MMBtu | $ / MMBTU	3.50
Fair Value | $	$ (4)
Sold Call Options - 2020 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	137
Cap price per MMBtu | $ / MMBTU	3.39
Fair Value | $	$ (21)
Sold Call Options - 2021 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	114
Cap price per MMBtu | $ / MMBTU	3.33
Fair Value | $	$ (27)
Sold Call Options [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume	350
Fair Value | $	$ (53)
Storage Fixed Price Swaps - 2018 [Member]
Derivative [Line Items]
Volume	1
Average price | $ / MMBTU	2.76
Storage Basis Swaps - 2018 [Member]
Derivative [Line Items]
Volume	1
Basis differential per MMBtu | $ / MMBTU	(0.88)
Financial protection on storage - 2018 [Member]
Derivative [Line Items]
Volume	2
Storage Fixed Price Swaps -2019 [Member]
Derivative [Line Items]
Volume	1
Average price | $ / MMBTU	3.03
Storage Basis Swaps - 2019 [Member]
Derivative [Line Items]
Volume	1
Basis differential per MMBtu | $ / MMBTU	(0.44)
Financial protection on storage - 2019 [Member]
Derivative [Line Items]
Volume	2