Income Taxes - Deferred Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 674	$ 1,297
Pension, postretirement and other benefits	2,435	2,544
Alternative minimum tax credit carryforward	189	379
Deferred revenue	1,620	1,416
Operating lease liabilities	1,579	0
Other	357	728
Valuation allowance	(13)	(15)
Total deferred tax assets	6,841	6,349
Deferred tax liabilities:
Depreciation	4,185	3,847
Operating lease right-of-use assets	1,388	0
Intangible assets	1,052	1,043
Other	137	105
Total deferred tax liabilities	6,762	4,995
Net deferred tax assets	79	1,354
Net state deferred tax assets	242	$ 1,354
Net federal deferred tax liabilities	$ 163