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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic EPS:
Net income	$ 1,047	$ 1,018	$ 819	$ 711	$ 836	$ 798	$ 696	$ 656	$ 3,595	$ 2,986	$ 2,821
Income allocated to RSUs									(45)	(43)	(44)
Income allocated to common stock for basic EPS calculation									$ 3,550	$ 2,943	$ 2,777
Weighted average number of shares outstanding, basic (in shares)									1,003	1,030	1,065
Basic earnings per common share	$ 1.04	$ 1.00	$ 0.81	$ 0.70	$ 0.81	$ 0.77	$ 0.66	$ 0.62	$ 3.54	$ 2.86	$ 2.61
Adjustment for dilutive shares:
Stock compensation plans									18	13	15
Diluted EPS:
Net income	$ 1,047	$ 1,018	$ 819	$ 711	$ 836	$ 798	$ 696	$ 656	$ 3,595	$ 2,986	$ 2,821
Income allocated to RSUs									(44)	(42)	(43)
Income allocated to common stock for diluted EPS									$ 3,551	$ 2,944	$ 2,778
Weighted average number of shares outstanding, diluted (in shares)									1,021	1,043	1,080
Diluted earnings per common share	$ 1.02	$ 0.98	$ 0.79	$ 0.69	$ 0.80	$ 0.76	$ 0.65	$ 0.61	$ 3.48	$ 2.82	$ 2.57