Equity Method Investments - Summarized Financial Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Information
Current Assets	$ 387	$ 390
Noncurrent Assets	575	588
Current Liabilities	198	171
Noncurrent Liabilities	81	90
Statements of Operations Information
Operating Revenues	855	790	$ 667
Operating Expenses	284	303	355
Operating Income	571	487	312
Other Income, net	3	15	7
Income Before Income Taxes	574	502	319
Income Tax Provision	152	136	60
Net Income	$ 422	$ 366	$ 259