Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Effective Income Tax Rate Reconciliation, Tax Settlement, Amount	$ (587)	[1]	$ 371	$ 412		$ (371)	[1]	$ (412)	[1]
Net income tax payments/(refunds)					$ 37	837		1,326
Unrecognized tax benefits that would affect the effective tax rate, if recognized	734		1,287		734	1,287		1,405
Unrecognized tax benefits	$ 966		$ 1,476		$ 966	$ 1,476		$ 2,412		$ 1,736

[1] In the fourth quarter of 2020, we recorded a tax benefit of $587 related to the settlement of the 2015-2017 federal tax audit. In the fourth quarter of 2019, we recorded a tax benefit of $371 related to the settlement of state tax audits spanning 15 tax years. In the third quarter of 2018, we recorded a tax benefit of $412 related to the settlement of the 2013-2014 federal tax audit.