Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		$ 35,000,000	$ 17,000,000	$ 18,000,000
Purchases		59,000,000	82,000,000	35,000,000
Settlements		(59,000,000)	(97,000,000)	(89,000,000)
(Losses) gains recognized in earnings	[1]	(1,000,000)	5,000,000	0
Gains (losses) recognized as regulatory assets and liabilities		(5,000,000)	28,000,000	53,000,000
Balance at end of period		29,000,000	35,000,000	17,000,000
Transfers into Level 3		0	0	0
Transfers out of Level 3		$ 0	$ 0	$ 0

[1] Amounts relate to commodity derivatives held at the end of the period.