Income Taxes - Components of Net Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets
Post-retirement and pension benefit costs	$ 1,164	$ 1,169
Net operating loss carryforwards	3,138	3,167
Other employee benefits	119	134
Other	604	577
Gross deferred tax assets	5,025	5,047
Less valuation allowance	(1,538)	(1,319)
Net deferred tax assets	3,487	3,728
Deferred tax liabilities
Property, plant and equipment, primarily due to depreciation differences	(3,882)	(3,489)
Goodwill and other intangible assets	(2,755)	(3,019)
Gross deferred tax liabilities	(6,637)	(6,508)
Net deferred tax liability	$ (3,150)	$ (2,780)