Fair Value Measurements (Reconciliations for Change in Net Fair Value of Derivative Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ (195)	$ 3
Included in earnings	199	(162)
Settlements	18	(36)
Transfers into/out of Level 3
Balance at end of period	22	(195)
Change in gains (losses) included in earnings relating to derivatives still held as of December 31	217	$ (198)
Sold Call Options [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amortization of premium paid	$ 5