Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

The weighted-average assumptions used in the option valuation model and the resulting weighted-average grant date fair values of stock options granted were as follows:

	Years ended December 31,
	2017			2016
Closing price of our common stock on grant date	$	162.60		$	156.35
Expected volatility (average of implied and historical volatility)	22.7		%	24.3		%
Expected life (in years)	5.8			5.8
Risk-free interest rate	2.1		%	1.5		%
Expected dividend yield	2.8		%	2.6		%
Fair value of stock options granted	$	27.54		$	27.55

Components of stock-based compensation expense recognized in the Consolidated Statements of Income

The following table reflects the components of stock-based compensation expense recognized in our Consolidated Statements of Income (in millions):

	Years ended December 31,
	2017			2016			2015
RSUs	$	174		$	177		$	190
Performance units	133			123			132
Stock options	22			11			—
Total stock-based compensation expense, pretax	329			311			322
Tax benefit from stock-based compensation expense	(118		)	(112		)	(120		)
Total stock-based compensation expense, net of tax	$	211		$	199		$	202

Summary of RSUs

The following table summarizes information regarding our RSUs:

	Year ended December 31, 2017
	Units (in millions)		Weighted-average grant date fair value
Balance nonvested at December 31, 2016	3.9		$	141.07
Granted	1.3		$	163.99
Vested	(1.5	)	$	125.32
Forfeited	(0.3	)	$	149.79
Balance nonvested at December 31, 2017	3.4		$	155.11

Weighted average assumptions used and the resulting weighted average grant date fair value of performance units

The weighted-average assumptions used in this model and the resulting weighted-average grant date fair values of performance units granted were as follows:

	Years ended December 31,
	2017			2016			2015
Closing price of our common stock on grant date	$	162.60		$	156.35		$	164.26
Volatility	25.9		%	25.8		%	24.3		%
Risk-free interest rate	1.4		%	0.9		%	0.8		%
Fair value of units granted	$	178.87		$	170.56		$	182.55

Summary of stock options

The following table summarizes information regarding our stock options:

	Year ended December 31, 2017
	Options (in millions)		Weighted- average exercise price		Weighted- average remaining contractual life (in years)	Aggregate intrinsic value (in millions)
Balance unexercised at December 31, 2016	3.1		$	100.21
Granted	1.5		$	162.60
Exercised	(0.5	)	$	57.24
Expired/forfeited	(0.1	)	$	159.13
Balance unexercised at December 31, 2017	4.0		$	127.08	6.9	$	186
Vested or expected to vest at December 31, 2017	3.7		$	124.84	6.8	$	182
Exercisable at December 31, 2017	1.3		$	58.23	2.8	$	147