Shareholders' Equity and Dividend Availability (details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Apr. 30, 2017
Shareholders' Equity and Dividend Availability disclosure
Number of authorized shares (in shares)	1,755.0				1,755.0
Number of authorized shares, preferred shares (in shares)	5.0				5.0
Number of authorized shares, voting common stock (in shares)	1,745.0				1,745.0
Number of authorized shares, undesignated shares (in shares)	5.0				5.0
Additional share repurchase authorization							$ 5,000
Number of shares purchased (in shares)	2.6	2.6	3.8	1.9	10.9
Cost of shares repurchased	$ 350	$ 328	$ 475	$ 225	$ 1,378
Average price paid per share (in dollars per share)	$ 133.70	$ 128.11	$ 123.04	$ 120.86	$ 126.42
Remaining capacity under share repurchase authorization	$ 4,556	$ 4,906	$ 5,234	$ 709	$ 4,556
Cost of shares acquired to cover tax withholding costs and exercise costs					$ 62	$ 72