Accounting Policies - Use of Estimates (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Property, Plant and Equipment [Line Items]
Decrease in depreciation and amortization	$ (5,362)	$ (4,854)	$ (22,297)		$ (16,524)
Increase in net income	$ 2,535	$ 3,561	$ 10,563		$ 12,005
Increase in basic earnings per share	$ 5.09	$ 7.24
Increase in diluted earnings per share	$ 5.01	$ 7.09
Technology equipment
Property, Plant and Equipment [Line Items]
Useful life of equipment	4 years			3 years
Change in equipment useful life
Property, Plant and Equipment [Line Items]
Decrease in depreciation and amortization	$ 786
Increase in net income	$ 602
Increase in basic earnings per share	$ 1.21
Increase in diluted earnings per share	$ 1.19