Basis of Presentation and Significant Accounting Policies and Practices - Earnings Per Share (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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic EPS:
Net income	$ 344	$ 1,285	$ 1,056	$ 997	$ 1,047	$ 1,018	$ 819	$ 711	$ 3,682	$ 3,595	$ 2,986
Income allocated to RSUs									(34)	(45)	(43)
Income allocated to common stock for basic EPS									$ 3,648	$ 3,550	$ 2,943
Weighted average number of shares outstanding, basic (in shares)									991	1,003	1,030
Basic earnings per common share	$ 0.35	$ 1.29	$ 1.05	$ 0.99	$ 1.04	$ 1.00	$ 0.81	$ 0.70	$ 3.68	$ 3.54	$ 2.86
Adjustment for dilutive shares:
Stock compensation plans									21	18	13
Diluted EPS:
Net income	$ 344	$ 1,285	$ 1,056	$ 997	$ 1,047	$ 1,018	$ 819	$ 711	$ 3,682	$ 3,595	$ 2,986
Income allocated to RSUs									(33)	(44)	(42)
Income allocated to common stock for diluted EPS									$ 3,649	$ 3,551	$ 2,944
Weighted average number of shares outstanding, diluted (in shares)									1,012	1,021	1,043
Diluted earnings per common share	$ 0.34	$ 1.26	$ 1.03	$ 0.97	$ 1.02	$ 0.98	$ 0.79	$ 0.69	$ 3.61	$ 3.48	$ 2.82