Schedule II - Consolidated Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule II - Consolidated Valuation And Qualifying Accounts [Abstract]
Allowance for uncollectible accounts, Balance at Beginning of Period	[1]	$ 58	$ 54	$ 66
Allowance for uncollectible accounts, Charged to Costs and Expenses	[1]	55	50	43
Allowance for uncollectible accounts, Deductions	[1],[2]	49	46	55
Allowance for uncollectible accounts, Balance at End of Period	[1]	$ 64	$ 58	$ 54

[1] Allowance for uncollectible accounts is deducted from Accounts receivable - Customers.

[2] Deductions consist principally of write-offs, net of collections of receivables previously written off.