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
	Mar. 31, 2019	Mar. 31, 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	(9)
Settlements	0	9
Transfers into/out of Level 3	0	0
Balance at end of period	0	22
Change in gains (losses) included in earnings relating to derivatives still held as of March 31	$ 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