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. This stock repurchase authorization replaced the previous $2.0 billion stock repurchase authorization, approved by the Board of Directors in 2010. There were no repurchases of common stock in Q3 2016 or Q3 2017.

Stock Award Activity

Common shares outstanding plus shares underlying outstanding stock awards totaled 497 million and 503 million as of December 31, 2016, and September 30, 2017. 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,
	2016		2017		2016		2017
Cost of sales (1)	$	7	$	13	$	7	$	33
Fulfillment	165		230		467		655
Marketing	85		135		221		363
Technology and content	434		595		1,171		1,668
General and administrative	85		112		222		317
Total stock-based compensation expense	$	776	$	1,085	$	2,088	$	3,036

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(1)	Beginning in Q3 2016, stock-based compensation expense was recorded to cost of sales for eligible employees providing delivery services.

The following table summarizes our restricted stock unit activity for the nine months ended September 30, 2017 (in millions):



	Number of Units		Weighted-Average Grant-Date Fair Value
Outstanding as of December 31, 2016	19.8		$	506
Units granted	7.2		918
Units vested	(4.6	)	387
Units forfeited	(1.3	)	620
Outstanding as of September 30, 2017	21.1		$	666

Scheduled vesting for outstanding restricted stock units as of September 30, 2017, is as follows (in millions):



	Three Months Ended December 31,	Year Ended December 31,
	2017	2018	2019	2020	2021	Thereafter	Total
Scheduled vesting—restricted stock units	2.2	7.3	7.0	3.1	1.1	0.4	21.1

As of September 30, 2017, there was $6.3 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, 2016 and September 30, 2017, was 28%. Changes in our estimates and assumptions relating to forfeitures may cause us to realize material changes in stock-based compensation expense in the future.