Related Party Transactions (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Cash distributions	$ 1,100	$ 625
Related Party Transaction Revenue with Affiliates	34	37
Related Party Transaction Expenditures from Transactions with Related Party	89	79
Equity Method Investments and Joint Ventures [Abstract]
Ownership Percentage in TTX Company			17.30%
Investments related to TTX	570		$ 554
Parent Company
Related Party Transaction [Line Items]
Cash distributions	1,100	625
Tax payments to parent company	1	$ 1
Receivable from parent company	43		235
Payable to parent company	26		19
Earliest Tax Year [Member] | Parent Company
Related Party Transaction [Line Items]
Tax payable to parent related to prior year tax audits	$ 32		$ 32