Special Charges and Mark-to-Market ("MTM") Adjustments - Components of Special Charges (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018		Jun. 30, 2019	Jun. 30, 2018
Operating:
Impairment of assets	$ 61	$ 111		$ 69	$ 134
Severance and benefit costs	6	11		12	25
(Gains) losses on sale of assets and other special charges	4	7		8	10
Total operating special charges	71	129	[1]	89	169	[1]
Nonoperating MTM (gains) losses on financial instruments	(34)	135		(51)	90
Total special charges and MTM (gains) losses on financial instruments	37	264		38	259
Income tax benefit	(8)	(59)		(8)	(58)
Total special charges and MTM (gains) losses on financial instruments, net of income tax	$ 29	$ 205		$ 30	$ 201

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.