Derivatives And Risk Management (Balance Sheet Classification Of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Call Options - 2017 [Member]
Derivatives, Fair Value [Line Items]
Premium paid	$ 3	$ 3
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	146	146	$ 155
Derivative liabilities	171	171	530
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	12	12
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	1	1	1
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	14	14	175
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	2	2	3
Not Designated as Hedging Instrument [Member] | Two-way Costless-collars [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	7	7	8
Not Designated as Hedging Instrument [Member] | Two-way Costless-collars [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets			2
Not Designated as Hedging Instrument [Member] | Two-way Costless-collars [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	7	7	49
Not Designated as Hedging Instrument [Member] | Two-way Costless-collars [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities			9
Not Designated as Hedging Instrument [Member] | Three-way Costless-collars [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	50	50	11
Not Designated as Hedging Instrument [Member] | Three-way Costless-collars [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	56	56	100
Not Designated as Hedging Instrument [Member] | Three-way Costless-collars [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	44	44	70
Not Designated as Hedging Instrument [Member] | Three-way Costless-collars [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	56	56	122
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	19	19	32
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets			1
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	23	23	13
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities			5
Not Designated as Hedging Instrument [Member] | Call Options [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	1	1
Not Designated as Hedging Instrument [Member] | Call Options [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	8	8	46
Not Designated as Hedging Instrument [Member] | Call Options [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	15	15	35
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	1	2
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ 1	1	1
Not Designated as Hedging Instrument [Member] | Call Options - 2017 [Member]
Derivatives, Fair Value [Line Items]
Premium paid		$ 2	$ 2