STOCKHOLDERS’ EQUITY

Preferred Stock

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

Common Stock

Common shares outstanding plus shares underlying outstanding stock awards totaled 490 million, 497 million, and 504 million, as of December 31, 2015, 2016, and 2017. 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. 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 2015, 2016, or 2017.

Stock Award Plans

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

Stock Award Activity

Stock options outstanding, which were primarily obtained through acquisitions, totaled 0.2 million, 0.1 million, and 0.1 million, as of December 31, 2015, 2016, and 2017.

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



	Year Ended December 31,
	2015		2016		2017
Cost of sales (1)	$	—	$	16	$	47
Fulfillment	482		657		911
Marketing	190		323		511
Technology and content	1,224		1,664		2,305
General and administrative	223		315		441
Total stock-based compensation expense (2)	$	2,119	$	2,975	$	4,215

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


(2)	The related tax benefits were $593 million, $907 million, and $860 million for 2015, 2016, and 2017. In 2017, the tax benefit reflects the permanent reduction in the U.S. statutory corporate tax rate from 35% to 21%.

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, 2015	17.4		$	285
Units granted	9.8		426
Units vested	(5.6	)	253
Units forfeited	(2.7	)	321
Outstanding as of December 31, 2015	18.9		362
Units granted	9.3		660
Units vested	(6.1	)	321
Units forfeited	(2.3	)	440
Outstanding as of December 31, 2016	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

Scheduled vesting for outstanding restricted stock units as of December 31, 2017, is as follows (in millions):



	Year Ended
	2018	2019	2020	2021	2022	Thereafter	Total
Scheduled vesting—restricted stock units	7.3	7.3	3.6	1.6	0.1	0.2	20.1

As of December 31, 2017, there was $6.4 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, 2015, 2016, and 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.

During 2015, 2016, and 2017, the fair value of restricted stock units that vested was $2.7 billion, $4.3 billion, and $6.8 billion.

As matching contributions under our 401(k) savings plan, we granted 0.1 million shares of common stock in 2016 and 2017. Shares granted as matching contributions under our 401(k) plan are included in outstanding common stock when issued, and recorded as stock-based compensation expense.

Common Stock Available for Future Issuance

As of December 31, 2017, common stock available for future issuance to employees is 116 million shares.