Changes in Accounting Principles Changes in Accounting Principles (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income taxes reclassified to retained earnings	$ 89
Cumulative effect of accounting changes	(9)				$ 49
Income tax expense		$ 801	$ 1,572	$ (1,693)
Right-of-use assets	1,415	1,312
Lease liability	1,415	$ 1,261
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting changes	81				$ 36
Accounting Standards Update 2016-13 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting changes	9
Income tax expense	$ 3