Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

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,
	2019			2018			2017
RSUs	$	168		$	165		$	174
Performance units	105			117			133
Stock options	35			29			22
Total stock-based compensation expense, pretax	308			311			329
Tax benefit from stock-based compensation expense	(67		)	(67		)	(118		)
Total stock-based compensation expense, net of tax	$	241		$	244		$	211

Summary of RSUs

The following table summarizes information regarding our RSUs:



	Year ended December 31, 2019
	Units (in millions)		Weighted-average grant date fair value
Balance nonvested as of December 31, 2018	3.1		$	168.11
Granted	1.2		$	182.12
Vested	(1.0	)	$	163.21
Forfeited	(0.2	)	$	170.52
Balance nonvested as of December 31, 2019	3.1		$	174.97

Weighted-average assumptions used and the resulting weighted-average grant date fair value of stock options

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,
	2019			2018			2017
Closing price of our common stock on grant date	$	177.31		$	177.46		$	162.60
Expected volatility (average of implied and historical volatility)	23.5		%	24.6		%	22.7		%
Expected life (in years)	5.8			5.8			5.8
Risk-free interest rate	2.4		%	2.8		%	2.1		%
Expected dividend yield	3.1		%	2.9		%	2.8		%
Fair value of stock options granted	$	30.47		$	34.60		$	27.54

Summary of stock options

The following table summarizes information regarding our stock options:



	Year ended December 31, 2019
	Options (in millions)		Weighted- average exercise price		Weighted- average remaining contractual life (in years)	Aggregate intrinsic value (in millions)
Balance unexercised as of December 31, 2018	4.4		$	143.57
Granted	1.4		$	177.31
Exercised	(0.7	)	$	107.13
Expired/forfeited	(0.3	)	$	171.01
Balance unexercised as of December 31, 2019	4.8		$	157.00	7.2	$	406
Vested or expected to vest as of December 31, 2019	4.6		$	156.02	7.1	$	390
Exercisable as of December 31, 2019	1.3		$	117.13	4.3	$	162

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

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



	Years ended December 31,
	2019			2018			2017
Closing price of our common stock on grant date	$	177.31		$	177.93		$	162.60
Volatility	22.1		%	23.8		%	25.9		%
Risk-free interest rate	2.3		%	2.6		%	1.4		%
Fair value of units granted	$	188.40		$	189.21		$	178.87