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, 2018	Dec. 31, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of year	$ 22	$ (195)
Included in earnings	(17)	199
Settlements	1	18
Transfers into/out of Level 3	6
Balance at end of period		22
Change in gains (losses) included in earnings relating to derivatives still held as of December 31		217
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		5
Natural Gas [Member] | Purchased Call Options [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amortization of premium paid	$ 1	$ 1