Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2015

Disclosure of Compensation Related Costs, Share-based Payments [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,
	2015			2014			2013
RSUs	$	190		$	219		$	206
Performance units	132			171			163
Stock options	—			18			34
Total stock-based compensation expense, pretax	322			408			403
Tax benefit from stock-based compensation expense	(120		)	(152		)	(149		)
Total stock-based compensation expense, net of tax	$	202		$	256		$	254

Summary of RSUs

The following summarizes select information regarding our RSUs:

	During the year ended December 31, 2015
	Units (in millions)		Weighted-average grant date fair value
Balance nonvested at December 31, 2014	7.1		$	92.88
Granted	1.4		$	166.74
Vested	(2.7	)	$	76.85
Forfeited	(0.8	)	$	112.53
Balance nonvested at December 31, 2015	5.0		$	118.89

Summary of stock options

The following summarizes select information regarding our stock options:

	During the year ended December 31, 2015
	Options (in millions)		Weighted- average exercise price		Weighted- average remaining contractual life (years)	Aggregate intrinsic value (in millions)
Balance unexercised at December 31, 2014	4.1		$	54.48
Granted	—		$	—
Exercised	(1.3	)	$	50.99
Expired/forfeited	—		$	51.96
Balance unexercised at December 31, 2015	2.8		$	56.19	3.8	$	294
Vested or expected to vest at December 31, 2015	2.8		$	56.19	3.8	$	294
Exercisable at December 31, 2015	2.8		$	56.19	3.8	$	294

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

The weighted-average assumptions used in these models and the resulting weighted-average grant date fair values of our performance units were as follows:

	Years ended December 31,
	2015			2014			2013
Closing price of our common stock on grant date	$	164.26		$	112.43		$	92.03
Volatility	24.3		%	23.8		%	21.0		%
Risk-free interest rate	0.8		%	0.8		%	0.4		%
Fair value of unit	$	182.55		$	104.47		$	102.73