Income Taxes - Deferred Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 1,495	$ 560
Capital loss carryforward	483	0
Pension, postretirement and other benefits	2,956	2,241
Deferred revenue	1,797	1,667
Lease liabilities	2,185	1,510
Other	611	380
Valuation allowance	(460)	(58)
Total deferred tax assets	9,067	6,300
Deferred tax liabilities:
Depreciation	4,507	5,190
Operating lease assets	1,324	1,298
Intangible assets	1,076	1,049
Other	172	99
Total deferred tax liabilities	7,079	7,636
Net deferred tax assets (liabilities)	1,988
Net deferred tax assets (liabilities)		(1,336)
Net state deferred tax assets		120
Net federal deferred tax liabilities	$ 0	$ 1,456