Stockholders' Equity

9 Months Ended

Sep. 30, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

STOCKHOLDERS’ EQUITY

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 during the nine months ended September 30, 2017 or 2018.

Stock Award Activity

Common shares outstanding plus shares underlying outstanding stock awards totaled 504 million and 507 million as of December 31, 2017 and September 30, 2018. These totals include all vested and unvested stock awards outstanding, including those awards we estimate will be forfeited. Stock-based compensation expense is as follows (in millions):



	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2018		2017		2018
Cost of sales	$	13	$	19	$	33	$	52
Fulfillment	230		269		655		834
Marketing	135		201		363		552
Technology and content	595		719		1,668		2,138
General and administrative	112		142		317		426
Total stock-based compensation expense	$	1,085	$	1,350	$	3,036	$	4,002

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



	Number of Units		Weighted-Average Grant-Date Fair Value
Outstanding as of December 31, 2017	20.1		$	725
Units granted	4.4		1,512
Units vested	(4.8	)	551
Units forfeited	(1.7	)	831
Outstanding as of September 30, 2018	18.0		$	954

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



	Three Months Ended December 31,	Year Ended December 31,
	2018	2019	2020	2021	2022	Thereafter	Total
Scheduled vesting—restricted stock units	2.3	7.0	5.6	2.3	0.6	0.2	18.0

As of September 30, 2018, there was $7.3 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 weighted-average recognition period of 1.1 years. The estimated forfeiture rate as of December 31, 2017 and September 30, 2018 was 28% and 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.