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, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	$ 2	$ 146
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(9)	(30)
Total gain (loss) on derivatives	(7)	116
Two-Way Costless Collars [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	3	31
Derivative Instrument, Gain (Loss) on Derivatives, Settled	4	(3)
Three-Way Costless Collars [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	(5)	57
Derivative Instrument, Gain (Loss) on Derivatives, Settled	7	(4)
Basis Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	20	(103)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(21)	(1)
Purchased Call Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	16
Derivative Instrument, Gain (Loss) on Derivatives, Settled	2
Sold Call Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	(34)	42
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(1)	(6)
Amortization of premium paid	1
Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	2	1
Purchased Call Options - 2018 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amortization of premium paid	1
Natural Gas [Member] | Fixed Price Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	(3)	118
Derivative Instrument, Gain (Loss) on Derivatives, Settled		(16)
Propane [Member] | Fixed Price Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	$ 3