Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Stock-Based Compensation Expense

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

	Year Ended December 31,
	2017		2018		2019
Cost of sales	$	47	$	73	$	149
Fulfillment	911		1,121		1,182
Technology and content	2,305		2,888		3,725
Marketing	511		769		1,135
General and administrative	441		567		673
Total stock-based compensation expense (1)	$	4,215	$	5,418	$	6,864

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(1)

The related tax benefits were $860 million, $1.1 billion, and $1.4 billion for 2017, 2018, and 2019.

Summary of Restricted Stock Unit Activity

The following table summarizes our restricted stock unit activity (in millions):

	Number of Units		Weighted Average Grant-Date Fair Value
Outstanding as of January 1, 2017	19.8		$	506
Units granted	8.9		946
Units vested	(6.8	)	400
Units forfeited	(1.8	)	649
Outstanding as of December 31, 2017	20.1		725
Units granted	5.0		1,522
Units vested	(7.1	)	578
Units forfeited	(2.1	)	862
Outstanding as of December 31, 2018	15.9		1,024
Units granted	6.7		1,808
Units vested	(6.6	)	827
Units forfeited	(1.7	)	1,223
Outstanding as of December 31, 2019	14.3		1,458

Scheduled Vesting of Outstanding Restricted Stock Units

Scheduled vesting for outstanding restricted stock units as of December 31, 2019, is as follows (in millions):

	Year Ended
	2020	2021	2022	2023	2024	Thereafter	Total
Scheduled vesting — restricted stock units	6.0	5.1	2.1	1.0	—	0.1	14.3