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	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) $ / MMBTU	Sep. 30, 2017 USD ($) $ / MMBTU ft³	Dec. 31, 2016 USD ($)
Fixed Price Swaps - 2017 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		73
Average price per MMBtu	3.06	3.06
Fair Value | $	$ 1	$ 1
Two-way Costless-collars - 2017 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		31
Fair Value | $	$ 1	$ 1
Two-way Costless-collars - 2017 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu	2.96	2.96
Two-way Costless-collars - 2017 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Cap price per MMBtu	3.38	3.38
Three-way Costless-collars - 2017 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		34
Three-way Costless-collars - 2017 Sold Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu	2.29	2.29
Three-way Costless-collars - 2017 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu	2.97	2.97
Three-way Costless-collars - 2017 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Cap price per MMBtu	3.30	3.30
Financial protection on production - 2017 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		138
Fair Value | $	$ 2	$ 2
Fixed Price Swaps - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		178
Average price per MMBtu	3.02	3.02
Fair Value | $	$ (4)	$ (4)
Two-way Costless-collars - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		23
Fair Value | $	$ (1)	$ (1)
Two-way Costless-collars - 2018 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu	2.97	2.97
Two-way Costless-collars - 2018 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Cap price per MMBtu	3.56	3.56
Three-way Costless-collars - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		272
Fair Value | $	$ 7	$ 7
Three-way Costless-collars - 2018 Sold Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu	2.40	2.40
Three-way Costless-collars - 2018 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu	2.97	2.97
Three-way Costless-collars - 2018 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Cap price per MMBtu	3.37	3.37
Financial protection on production - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		473
Fair Value | $	$ 2	$ 2
Three-way Costless-collars - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		108
Fair Value | $	$ (1)	$ (1)
Three-way Costless-collars - 2019 Sold Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu	2.50	2.50
Three-way Costless-collars - 2019 Purchased Puts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Floor price per MMBtu	2.95	2.95
Three-way Costless-collars - 2019 Sold Calls [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Cap price per MMBtu	3.32	3.32
Financial protection on production - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		108
Fair Value | $	$ (1)	$ (1)
Basis Swaps - 2017 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		32
Basis differential per MMBtu	(0.95)	(0.95)
Fair Value | $	$ 12	$ 12
Basis Swaps - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		25
Basis differential per MMBtu	(0.63)	(0.63)
Fair Value | $	$ (16)	$ (16)
Basis Swaps [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		57
Fair Value | $	(4)	$ (4)
Call Options - 2017 [Member]
Derivative [Line Items]
Premium paid | $	$ 3	$ 3
Call Options - 2017 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		21
Cap price per MMBtu	3.68	3.68
Premium paid | $		$ 2	$ 2
Call Options - 2018 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		63
Cap price per MMBtu	3.50	3.50
Fair Value | $	$ (9)	$ (9)
Call Options - 2019 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		52
Cap price per MMBtu	3.50	3.50
Fair Value | $	$ (8)	$ (8)
Call Options - 2020 [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		32
Cap price per MMBtu	3.75	3.75
Fair Value | $	$ (5)	$ (5)
Call Options [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Volume (Bcf) | ft³		168
Fair Value | $	$ (22)	$ (22)