Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2018

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



	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2018		2017		2018
Cost of sales	$	13	$	19	$	33	$	52
Fulfillment	230		269		655		834
Marketing	135		201		363		552
Technology and content	595		719		1,668		2,138
General and administrative	112		142		317		426
Total stock-based compensation expense	$	1,085	$	1,350	$	3,036	$	4,002

The following table summarizes our restricted stock unit activity for the nine months ended September 30, 2018 (in millions):



	Number of Units		Weighted-Average Grant-Date Fair Value
Outstanding as of December 31, 2017	20.1		$	725
Units granted	4.4		1,512
Units vested	(4.8	)	551
Units forfeited	(1.7	)	831
Outstanding as of September 30, 2018	18.0		$	954

Scheduled vesting for outstanding restricted stock units as of September 30, 2018, is as follows (in millions):



	Three Months Ended December 31,	Year Ended December 31,
	2018	2019	2020	2021	2022	Thereafter	Total
Scheduled vesting—restricted stock units	2.3	7.0	5.6	2.3	0.6	0.2	18.0