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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	$ 118	$ (56)	$ 96	$ (54)
Settled Gain (Loss) on Derivatives Recognized in Earnings	34	20	24	11
Total gain (loss) on derivatives	152	(36)	120	(43)
Purchased fixed price swaps | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	1	0	5	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	(1)	0	(2)	0
Fixed price swaps | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	5	0	(8)	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	2	0	4	0
Fixed price swaps | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	57	(26)	55	(29)
Settled Gain (Loss) on Derivatives Recognized in Earnings	14	13	8	13
Fixed price swaps | Propane
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	13	(12)	9	(9)
Settled Gain (Loss) on Derivatives Recognized in Earnings	7	(1)	9	(1)
Fixed price swaps | Ethane
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	0	(2)	7	(2)
Settled Gain (Loss) on Derivatives Recognized in Earnings	5	0	6	0
Two-way costless collars | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	4	0	(3)	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	1	0	2	0
Two-way costless collars | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	10	(1)	9	2
Settled Gain (Loss) on Derivatives Recognized in Earnings	3	0	2	4
Two-way costless collars | Propane
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	2	0	2	0
Three-way costless collars | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	1	0	1	0
Three-way costless collars | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	22	(24)	24	(29)
Settled Gain (Loss) on Derivatives Recognized in Earnings	8	12	4	19
Basis swaps | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	4	(4)	(6)	16
Settled Gain (Loss) on Derivatives Recognized in Earnings	(4)	(3)	(8)	(24)
Purchased call options | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	(2)	(12)	(2)	4
Settled Gain (Loss) on Derivatives Recognized in Earnings	0	0	0	2
Amortization of premium paid				1
Sold call options | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	0	(6)	0	(6)
Settled Gain (Loss) on Derivatives Recognized in Earnings	0	(1)	0	(1)
Sold call options | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	4	31	6	(3)
Settled Gain (Loss) on Derivatives Recognized in Earnings	(1)	0	(1)	(1)
Storage - fixed price swap
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled 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	$ (2)	$ 0	$ (2)	$ 2