Accounting Principles and Practices - Schedule of Changes in Accounting Principle (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income	$ (16)	$ (127)	$ 783	$ 208
Other income (expense)	$ (3)	4	$ (18)	2
As Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income		(118)		225
Other income (expense)		(5)		(15)
Adjustments | Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income		(9)		(17)
Other income (expense)		$ 9		$ 17