Derivatives and Risk Management (Schedule of Derivative Instruments, Notional Amount in BCF, Weighted Average Contract Prices and Fair Value) (Details) $ in Millions, ft³ in Billions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / MMBTU ft³
Fixed Price Swaps - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	194
Average price per MMBtu | $ / MMBTU	3.02
Fair Value	$ 38
Two-way Costless-collars - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	23
Fair Value	$ 4
Two-way Costless-collars - 2018 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.97
Two-way Costless-collars - 2018 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	3.56
Three-way Costless-collars - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	272
Fair Value	$ 46
Three-way Costless-collars - 2018 Sold Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.40
Three-way Costless-collars - 2018 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.97
Three-way Costless-collars - 2018 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	3.37
Financial protection on production - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	489
Fair Value	$ 88
Fixed Price Swaps - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	93
Average price per MMBtu | $ / MMBTU	3.00
Fair Value	$ 17
Three-way Costless-collars - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	108
Fair Value	$ 9
Three-way Costless-collars - 2019 Sold Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.50
Three-way Costless-collars - 2019 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.95
Three-way Costless-collars - 2019 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	3.32
Financial protection on production - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	201
Fair Value	$ 26
Basis Swaps - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	44
Basis differential per MMBtu | $ / MMBTU	(0.48)
Fair Value	$ (21)
Basis Swaps - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Basis differential per MMBtu | $ / MMBTU	(0.59)
Basis Swaps [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	44
Fair Value	$ (21)
Sold Call Options - 2018 [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	63
Cap price per MMBtu | $ / MMBTU	3.50
Fair Value	$ (3)
Sold Call Options - 2019 [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	52
Cap price per MMBtu | $ / MMBTU	3.50
Fair Value	$ (5)
Sold Call Options - 2020 [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	68
Cap price per MMBtu | $ / MMBTU	3.63
Fair Value	$ (4)
Sold Call Options - 2021 [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	57
Cap price per MMBtu | $ / MMBTU	3.52
Fair Value	$ (6)
Sold Call Options [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	240
Fair Value	$ (18)
Purchased Call Options - 2018 [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	13
Cap price per MMBtu | $ / MMBTU	3.23
Fair Value	$ 2
Premium paid	$ 1
Purchased Call Options [Member]
Derivative [Line Items]
Volume (Bcf) | ft³	13
Fair Value	$ 2
Premium paid	$ 1