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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		$ 19,000,000	$ 6,000,000	$ 35,000,000	$ 17,000,000
Purchases		45,000,000	76,000,000	46,000,000	80,000,000
Settlements		(20,000,000)	(22,000,000)	(32,000,000)	(42,000,000)
Gains (losses) recognized in earnings	[1]	(2,000,000)	6,000,000	0	5,000,000
Net gains recognized as regulatory assets and liabilities		22,000,000	3,000,000	15,000,000	9,000,000
Balance at end of period		64,000,000	69,000,000	64,000,000	69,000,000
Transfers into Level 3		0	0	0	0
Transfers out of Level 3		$ 0	$ 0	$ 0	$ 0

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