Income Taxes - Tax Contingencies (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Liabilities associated with uncertain tax positions	$ 182	$ 164
Unrecognized tax benefits, income tax penalties and interest accrued	11	11
Noncurrent Deferred Tax Assets
Income Tax Contingency [Line Items]
Liabilities associated with uncertain tax positions	4	3
Other Taxes Payable
Income Tax Contingency [Line Items]
Liabilities associated with uncertain tax positions	$ 178	$ 161