Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2017

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



	Three Months Ended March 31,
	2016		2017
Cost of sales (1)	$	—	$	8
Fulfillment	116		163
Marketing	56		94
Technology and content	317		441
General and administrative	55		86
Total stock-based compensation expense	$	544	$	792

The following table summarizes our restricted stock unit activity for the three months ended March 31, 2017 (in millions):



	Number of Units		Weighted-Average Grant-Date Fair Value
Outstanding as of December 31, 2016	19.8		$	506
Units granted	1.2		844
Units vested	(1.1	)	335
Units forfeited	(0.4	)	541
Outstanding as of March 31, 2017	19.5		$	536

Scheduled vesting for outstanding restricted stock units as of March 31, 2017, is as follows (in millions):



	Nine Months Ended December 31,	Year Ended December 31,
	2017	2018	2019	2020	2021	Thereafter	Total
Scheduled vesting—restricted stock units	5.9	7.1	3.9	2.0	0.3	0.3	19.5