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	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 22	$ (168)	$ 22	$ (195)
Included in earnings	(8)	86	(17)	99
Settlements	(8)	30	1	44
Transfers into/out of Level 3	$ (6)		(6)
Balance at end of period		(52)		(52)
Change in gains (losses) included in earnings relating to derivatives still held as of June 30		$ 116		$ 143
Natural Gas [Member] | Sold Call Options [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amortization of premium paid			$ 1