Employee Incentive Plans - Stock Options (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Incentive Plan Aggregate Disclosures [Line Items]
Compensation cost	$ 178	$ 128	$ 111
Employee Stock Option [Member]
Employee Incentive Plan Aggregate Disclosures [Line Items]
Compensation cost	$ 52	$ 53	$ 42
Black-Scholes option-pricing model assumptions and resulting fair value of options [Abstract]
Dividend Yield	0.00%	0.00%	0.00%
Expected volatility	35.00%	35.00%	35.00%
Risk-free interest rate	1.80%	1.20%	1.30%
Expected option life	4 years 3 months 18 days	4 years 3 months 18 days	4 years 3 months 18 days
Weighted average fair value of options	$ 46.38	$ 42.01	$ 36.4
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding - January 1	7,097	6,433	7,331
Options Granted	1,230	1,336	1,410
Options Exercised	(2,072)	(577)	(2,146)
Options Expired or canceled	(99)	(95)	(162)
Options outstanding - December 31	6,156	7,097	6,433
Options Exercisable - Number (in thousands)				3,894	4,409	3,414
Share Based Compensation Arrangement By Share Based Payment Award, Options Outstanding, Weighted Average Exercise Price [Roll Forward]
Weighted Average Exercise Price - Options Outstanding - January 1	$ 82.01	$ 68.86	$ 51.84
Weighted Average Exercise Price - Options Granted	149.17	139.2	120.94
Weighted Average Exercise Price - Options Exercised	63.41	62.09	43.63
Weighted Average Exercise Price - Options Expired or canceled	138.41	117.18	86.04
Weighted Average Exercise Price - Options Outstanding - December 31	$ 100.79	$ 82.01	$ 68.86
Options Exercisable - Weighted Average Exercise Price				$ 77.36	$ 58.36	$ 46.55
Related compensation expense to be recognized				$ 39
Period over which compensation expense will be recognized	2 years
Information For Stock Options Exercised Details [Abstract]
Intrinsic value of options exercised	$ 218	$ 41	$ 179
Cash received for options exercised	131	36	94
Tax benefit from options exercised	$ 41	$ 11	$ 42
Information for Outstanding Common Stock Options [Abstract]
Options Outstanding - Number (in thousands)	6,156	7,097	7,331	6,156	7,097	6,433
Options Outstanding - Total intrinsic value				$ 630
Options Outstanding - Weighted Average Exercise Price	$ 82.01	$ 82.01	$ 51.84	$ 100.79	$ 82.01	$ 68.86
Options Outstanding - Weighted average remaining contractual life	6 years 7 months 6 days
Options Exercisable - Number (in thousands)				3,894	4,409	3,414
Options Exercisable - Total Intrinsic Value				$ 490
Options Exercisable - Weighted Average Exercise Price				$ 77.36	$ 58.36	$ 46.55
Options Exercisable - Weighted Average Remaining Contractual Life	5 years 6 months
Employee Stock Option [Member] | Minimum [Member]
Employee Incentive Plan Aggregate Disclosures [Line Items]
Award vesting period	1 year
Employee Stock Option [Member] | Maximum [Member]
Employee Incentive Plan Aggregate Disclosures [Line Items]
Award vesting period	3 years
Award expiration period	10 years
Black-Scholes option-pricing model assumptions and resulting fair value of options [Abstract]
Remaining maturity of traded options	1 year