Supplementary Financial Information (Trade Accounts Receivable and Allowance for Uncollectible Accounts Receivable) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Dec. 31, 2014
Trade Accounts Receivable [Abstract]
Wholesale and retail trade accounts receivable			$ 786	$ 603
Allowance for uncollectible accounts	$ (15)	$ (14)	(16)	(15)
Trade accounts receivable — net			770	588
Unbilled Receivables, Current			$ 287	$ 239
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance for uncollectible accounts receivable at beginning of period	15	14
Increase for bad debt expense	29	30
Decrease for account write-offs	(28)	(27)
Allowance for uncollectible accounts receivable at end of period	$ 16	$ 17