Fair Value Measurements (Level 3 Reconciliation) (Details) - Fair Value, Inputs, Level 3 [Member] - Price Risk Management Instruments [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Asset (liability) balance as of January 1		$ 55	$ 89
Net realized and unrealized gains: Included in regulatory assets and liabilities or balancing accounts	[1]	(13)	(34)
Asset (liability) balance as of December 31		$ 42	$ 55

[1] The costs related to price risk management activities are recoverable through customer rates, therefore, balancing account revenue is recorded for amounts settled and purchased and there is no impact to net income. Unrealized gains and losses are deferred in regulatory liabilities and assets.