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
	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	$ (22)	$ 2
Settled Gain (Loss) on Derivatives Recognized in Earnings	(10)	(9)
Total gain (loss) on derivatives	(32)	(7)
Purchased fixed price swaps | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	4	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	(1)	0
Fixed price swaps | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	(13)	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	2	0
Fixed price swaps | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	(2)	(3)
Settled Gain (Loss) on Derivatives Recognized in Earnings	(6)	0
Fixed price swaps | Propane
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	(4)	3
Settled Gain (Loss) on Derivatives Recognized in Earnings	2	0
Fixed price swaps | Ethane
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	7	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	1	0
Two-way costless collars | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	(7)	0
Settled Gain (Loss) on Derivatives Recognized in Earnings	1	0
Two-way costless collars | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	(1)	3
Settled Gain (Loss) on Derivatives Recognized in Earnings	(1)	4
Three-way costless collars | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	2	(5)
Settled Gain (Loss) on Derivatives Recognized in Earnings	(4)	7
Basis swaps | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	(10)	20
Settled Gain (Loss) on Derivatives Recognized in Earnings	(4)	(21)
Purchased call options | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	0	16
Settled Gain (Loss) on Derivatives Recognized in Earnings	0	2
Amortization of premium paid		1
Sold call options | Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	2	(34)
Settled Gain (Loss) on Derivatives Recognized in Earnings	0	(1)
Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Unsettled Gain (Loss) on Derivatives Recognized in Earnings	$ 0	$ 2