Stock Based Compensation (Tables)

Stock-based compensation expense included the following components:



	2016		2015		2014
	(millions)
Stock options	$	43	$	52	$	47
Restricted stock units	18		13		26
	$	61	$	65	$	73

The fair value of stock options granted during 2016, 2015 and 2014 and the weighted average assumptions used to estimate the fair value are as follows:



	2016			2015			2014
Weighted average grant date fair value of stock options granted during the period	$	12.14		$	20.78		$	19.07
Dividend yield	3.8		%	2.7		%	2.5		%
Expected volatility	42.7		%	43.3		%	42.7		%
Risk-free interest rate	1.4		%	1.7		%	1.5		%
Expected life	5.7 years			5.7 years			5.7 years

Activity related to stock options for 2016 is as follows:



	Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Life	Aggregate Intrinsic Value
	(thousands)				(years)	(millions)
Outstanding, beginning of period	18,829.8		$	41.92
Granted	3,886.8		$	42.97
Canceled or forfeited	(1,116.6	)	$	51.33
Exercised	(1,122.1	)	$	31.30
Outstanding, end of period	20,477.9		$	42.18
Exercisable, end of period	12,541.5		$	36.48	4.1	$	46
Options expected to vest	6,657.5		$	51.07	8.3	$	—

Additional information relating to stock options is as follows:



	2016		2015		2014
	(millions)
Intrinsic value of options exercised	$	12	$	127	$	189
Cash received from stock options exercised	35		125		200

Activity related to restricted stock units for 2016 is as follows:



	Shares		Weighted Average Grant Date Fair Value
	(thousands)
Nonvested, beginning of period	1,497.0		$	57.06
Granted – performance-based	575.1		43.72
Performance adjustment	(237.6	)	59.82
Granted – time-based	482.8		35.61
Forfeited	(250.0	)	32.99
Vested	(249.0	)	33.70
Nonvested, end of period	1,818.3		$	53.29

Restricted Stock Units

The weighted average grant date fair values of restricted stock units granted during 2016, 2015 and 2014 are as follows:



	2016		2015		2014
Restricted stock units	$	40.02	$	62.61	$	59.41