Income Taxes - Tax Credit Carryforward Table (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Foreign Tax Credits [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforward	$ 2,685
Foreign Tax Credits [Member] | Earliest Tax Year [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforward, expiration date	Dec. 31, 2023
Foreign Tax Credits [Member] | Latest Tax Year [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforward, expiration date	Dec. 31, 2028
Texas Margins Tax Credit [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforward	$ 30
Tax credit carryforward, expiration date	Dec. 31, 2026