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	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 0	$ 22
Included in earnings	0	(17)
Settlements	0	1
Transfers into/out of Level 3	0	(6)
Balance at end of period	0	0
Change in gains (losses) included in earnings relating to derivatives still held as of June 30	$ 0	0
Natural Gas | Call Option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Amortization of premium paid		$ 1