Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		$ (7,000,000)	$ 19,000,000	$ 29,000,000	$ 35,000,000
Purchases		34,000,000	45,000,000	38,000,000	46,000,000
Settlements		(16,000,000)	(20,000,000)	(27,000,000)	(32,000,000)
Gains (losses) recognized in earnings	[1]	7,000,000	(2,000,000)	(11,000,000)	0
Net gains recognized as regulatory assets and liabilities		10,000,000	22,000,000	(1,000,000)	15,000,000
Balance at end of period		28,000,000	64,000,000	28,000,000	64,000,000
Transfers into Level 3		0	0	0	0
Transfers out of Level 3		$ 0	$ 0	$ 0	$ 0

[1] ^(a)  These amounts relate to commodity derivatives held at the end of the period.