Derivatives and Risk Management (Summary of Before Tax Effect of Fair Value Hedges not Designated for Hedge Accounting) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	$ 12	$ (59)	$ 108	$ (113)
Settled Gain (Loss) on Derivatives Recognized in Earnings	88	(6)	112	5
Total gain (loss) on derivatives	100	(65)	220	(108)
Purchased fixed price swaps | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	0	0	5	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	0	0	(2)	0
Fixed price swaps | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	8	0	0	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	3	0	7	0
Fixed price swaps | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	(19)	(17)	36	(46)
Settled Gain (Loss) on Derivatives Recognized in Earnings	45	5	53	18
Fixed price swaps | Propane
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	10	(10)	19	(19)
Settled Gain (Loss) on Derivatives Recognized in Earnings	11	(5)	20	(6)
Fixed price swaps | Ethane
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	1	(27)	8	(29)
Settled Gain (Loss) on Derivatives Recognized in Earnings	6	(6)	12	(6)
Two-way costless collars | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	0	0	(3)	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	2	0	4	0
Two-way costless collars | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	(11)	2	(2)	4
Settled Gain (Loss) on Derivatives Recognized in Earnings	10	0	12	4
Two-way costless collars | Propane
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	1	0	3	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	1	0	1	0
Three-way costless collars | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	3	0	4	0
Three-way costless collars | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	3	(7)	27	(36)
Settled Gain (Loss) on Derivatives Recognized in Earnings	15	5	19	24
Basis swaps | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	12	(5)	6	11
Settled Gain (Loss) on Derivatives Recognized in Earnings	(3)	(4)	(11)	(28)
Purchased call options | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	(1)	(2)	(3)	2
Settled Gain (Loss) on Derivatives Recognized in Earnings	(1)	0	(1)	2
Amortization of premium paid	1		1	1
Sold call options | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	0	1	0	(5)
Settled Gain (Loss) on Derivatives Recognized in Earnings	0	(1)	0	(2)
Sold call options | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	2	5	8	2
Settled Gain (Loss) on Derivatives Recognized in Earnings	0	0	(1)	(1)
Storage - fixed price swap
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	2	0	1	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	(1)	0	(1)	0
Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	$ 1	$ 1	$ (1)	$ 3