Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
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
Purchases		1,000,000	4,000,000
Settlements		(12,000,000)	(20,000,000)
Gains (losses) recognized in earnings	[1]	2,000,000	0
Net gains recognized as regulatory assets and liabilities		(7,000,000)	5,000,000
Balance at end of period		19,000,000	6,000,000
Transfers into Level 3		0	0
Transfers out of Level 3		$ 0	$ 0

[1] 017: Three Months Ended March 31(Millions of Dollars) 2018 2017Balance at Jan. 1 $35 $17Purchases 1 4Settlements (12) (20)Net transactions recorded during the period: Gains recognized in earnings (a) 2 —Net (losses) gains recognized as regulatory assets and liabilities (7) 5Balance at March 31 $19 $6 (a) These amounts relate to commodity derivatives held at the end of the