Stockholders' Equity

6 Months Ended

Jun. 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 six months ended June 30, 2017 or 2018.

Stock Award Activity

Common shares outstanding plus shares underlying outstanding stock awards totaled 504 million and 506 million as of December 31, 2017 and June 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 June 30,				Six Months Ended June 30,
	2017		2018		2017		2018
Cost of sales	$	12	$	19	$	21	$	34
Fulfillment	261		320		425		564
Marketing	133		190		228		351
Technology and content	633		788		1,073		1,419
General and administrative	119		151		205		284
Total stock-based compensation expense	$	1,158	$	1,468	$	1,952	$	2,652

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



	Number of Units		Weighted-Average Grant-Date Fair Value
Outstanding as of December 31, 2017	20.1		$	725
Units granted	3.7		1,437
Units vested	(3.6	)	552
Units forfeited	(1.2	)	808
Outstanding as of June 30, 2018	19.0		$	891

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



	Six Months Ended December 31,	Year Ended December 31,
	2018	2019	2020	2021	2022	Thereafter	Total
Scheduled vesting—restricted stock units	3.5	7.1	5.6	2.1	0.4	0.3	19.0

As of June 30, 2018, there was $7.6 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 June 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.