Summary of Significant Accounting Policies - Various Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Foreign Currency Transactions
Gain (loss) on revaluation of foreign currency transactions and monetary assets and liabilities	$ 1	$ 21	$ (8)
Cash and Cash Equivalents
Outstanding checks included in accounts payable that were reclassified from cash and cash equivalents	57	60
Asset Retirement Obligations
Fair value of asset retirement obligations	109	103	44	$ 35
Other long-term liabilities and deferred credits
Asset Retirement Obligations
Fair value of asset retirement obligations	$ 107	$ 99	$ 33