Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2018

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



	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2018		2017		2018
Cost of sales	$	12	$	19	$	21	$	34
Fulfillment	261		320		425		564
Marketing	133		190		228		351
Technology and content	633		788		1,073		1,419
General and administrative	119		151		205		284
Total stock-based compensation expense	$	1,158	$	1,468	$	1,952	$	2,652

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



	Number of Units		Weighted-Average Grant-Date Fair Value
Outstanding as of December 31, 2017	20.1		$	725
Units granted	3.7		1,437
Units vested	(3.6	)	552
Units forfeited	(1.2	)	808
Outstanding as of June 30, 2018	19.0		$	891

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



	Six Months Ended December 31,	Year Ended December 31,
	2018	2019	2020	2021	2022	Thereafter	Total
Scheduled vesting—restricted stock units	3.5	7.1	5.6	2.1	0.4	0.3	19.0