Special Charges and Mark-to-Market Adjustments - Components of Special Charges (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017
Other Income and Expenses [Abstract]
Impairment of assets	$ 11	$ 15		$ 145	$ 15
Severance and benefit costs	9	23		34	101
(Gains) losses on sale of assets and other special charges	(3)	12		7	29
Total special charges	17	50	[1]	186	145	[1]
Nonoperating mark-to-market (MTM) (gains) losses on equity investments	(29)	0		61	0
Total special charges and MTM (gains) losses on equity investments	(12)	50		247	145
Income tax expense (benefit)	3	(18)		(55)	(52)
Total special charges and MTM (gains) losses on equity investments, net of tax	$ (9)	$ 32		$ 192	$ 93

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) and Accounting Standards Update No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.