Stockholders' Equity

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Stockholders' Equity

STOCKHOLDERS’ EQUITY

Preferred Stock

We have authorized 500 million shares of $0.01 par value preferred stock. No preferred stock was outstanding for any year presented.

Common Stock

Common shares outstanding plus shares underlying outstanding stock awards totaled 507 million, 512 million, and 518 million, as of December 31, 2018, 2019, and 2020. These totals include all vested and unvested stock awards outstanding, including those awards we estimate will be forfeited.

Stock Repurchase Activity

In February 2016, the Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock, with no fixed expiration. There were no repurchases of common stock in 2018, 2019, or 2020.

Stock Award Plans

Employees vest in restricted stock unit awards over the corresponding service term, generally between two and five years.

Stock Award Activity

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.

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 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

As of December 31, 2020, there was $13.3 billion of net unrecognized compensation cost related to unvested stock-based compensation arrangements. This compensation is recognized on an accelerated basis with approximately half of the compensation expected to be expensed in the next twelve months, and has a remaining weighted-average recognition period of 1.2 years. The estimated forfeiture rate as of December 31, 2018, 2019, and 2020 was 27%. Changes in our estimates and assumptions relating to forfeitures may cause us to realize material changes in stock-based compensation expense in the future.

During 2018, 2019, and 2020, the fair value of restricted stock units that vested was $11.4 billion, $11.7 billion, and $15.5 billion.

Common Stock Available for Future Issuance

As of December 31, 2020, common stock available for future issuance to employees is 102 million shares.