Derivatives and Risk Management (Summary of Before Tax Effect of Cash Flow Hedges on Consolidated Financial Statements) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	$ (24)	$ 451
Derivative Instrument, Settled Gain (Loss) on Derivatives	(94)	(29)
Total gain (loss) on derivatives	(118)	422
Purchased Fixed Price Swap [Member] | Oil [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	(6)
Fixed Price Swaps [Member] | Natural Gas [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	(27)	232
Derivative Instrument, Settled Gain (Loss) on Derivatives	(32)	(9)
Fixed Price Swaps [Member] | Oil [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	19
Fixed Price Swaps [Member] | Propane [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	11
Derivative Instrument, Settled Gain (Loss) on Derivatives	(6)
Fixed Price Swaps [Member] | Ethane [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	5
Derivative Instrument, Settled Gain (Loss) on Derivatives	(8)
Two-Way Costless Collars [Member] | Natural Gas [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives		52
Derivative Instrument, Settled Gain (Loss) on Derivatives	(1)
Two-Way Costless Collars [Member] | Oil [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	10
Three-Way Costless Collars [Member] | Natural Gas [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	(48)	136
Derivative Instrument, Settled Gain (Loss) on Derivatives	(9)	(1)
Basis Swaps [Member] | Natural Gas [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	10	(36)
Derivative Instrument, Settled Gain (Loss) on Derivatives	(31)	(6)
Purchased Call Options [Member] | Natural Gas [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	4	2
Derivative Instrument, Settled Gain (Loss) on Derivatives	2
Amortization of premium paid	1	1
Sold Call Options [Member] | Natural Gas [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	(4)	63
Derivative Instrument, Settled Gain (Loss) on Derivatives	(7)	(11)
Amortization of premium paid		5
Sold Call Options [Member] | Oil [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Settled Gain (Loss) on Derivatives	(2)
Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Unsettled Gain (Loss) on Derivatives	$ 2	2
Derivative Instrument, Settled Gain (Loss) on Derivatives		$ (2)