Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Stock-Based Compensation Expense

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

	Year Ended December 31,
	2018		2019		2020
Cost of sales	$	73	$	149	$	283
Fulfillment	1,121		1,182		1,357
Technology and content	2,888		3,725		5,061
Marketing	769		1,135		1,710
General and administrative	567		673		797
Total stock-based compensation expense (1)	$	5,418	$	6,864	$	9,208

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(1)The related tax benefits were $1.1 billion, $1.4 billion, and $1.9 billion for 2018, 2019, and 2020.

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, 2018	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
Units granted	8.0	2,373
Units vested	(5.8)	1,239
Units forfeited	(1.3)	1,642
Outstanding as of December 31, 2020	15.2	2,004

Scheduled Vesting of Outstanding Restricted Stock Units

Scheduled vesting for outstanding restricted stock units as of December 31, 2020, is as follows (in millions):

	Year Ended
	2021	2022	2023	2024	2025	Thereafter	Total
Scheduled vesting — restricted stock units	5.7	5.4	2.7	1.2	0.1	0.1	15.2