Derivatives and Risk Management (Summary of Before Tax Effect of Fair Value Hedges not Designated for Hedge Accounting) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	$ 451	$ (373)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(29)	34
Total gain (loss) on derivatives	422	(339)
Fixed Price Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	232	(177)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(9)
Purchased Put Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Settled		11
Two-way Costless-collars [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	52	(48)
Derivative Instrument, Gain (Loss) on Derivatives, Settled		3
Three-way Costless-collars [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	136	(81)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(1)	1
Basis Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	(36)	12
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(6)	21
Purchased Call Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	2
Sold Call Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	63	(81)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(11)
Amortization of premium paid	5
Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	2	2
Derivative Instrument, Gain (Loss) on Derivatives, Settled	$ (2)	$ (2)