Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2016

Stock-based compensation expense is as follows (in millions):



	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2015		2016		2015
Cost of sales (1)	$	7	$	—	$	7	$	—
Fulfillment	165		122		467		344
Marketing	85		48		221		133
Technology and content	434		309		1,171		861
General and administrative	85		65		222		175
Total stock-based compensation expense	$	776	$	544	$	2,088	$	1,513

The following table summarizes our restricted stock unit activity for the nine months ended September 30, 2016 (in millions):



	Number of Units		Weighted-Average Grant-Date Fair Value
Outstanding as of December 31, 2015	18.9		$	362
Units granted	7.9		640
Units vested	(4.2	)	309
Units forfeited	(1.8	)	420
Outstanding as of September 30, 2016	20.8		$	473

Scheduled vesting for outstanding restricted stock units as of September 30, 2016, is as follows (in millions):



	Three Months Ended December 31,	Year Ended December 31,
	2016	2017	2018	2019	2020	Thereafter	Total
Scheduled vesting—restricted stock units	2.0	7.0	7.0	3.1	1.2	0.5	20.8