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 in Q1 2018 or Q1 2019.

Stock Award Activity

Common shares outstanding plus shares underlying outstanding stock awards totaled 507 million as of December 31, 2018 and March 31, 2019. 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 March 31,
	2018		2019
Cost of sales	$	15	$	24
Fulfillment	244		234
Marketing	161		209
Technology and content	631		675
General and administrative	132		132
Total stock-based compensation expense	$	1,183	$	1,274

The following table summarizes our restricted stock unit activity for the three months ended March 31, 2019 (in millions):



	Number of Units		Weighted-Average Grant-Date Fair Value
Outstanding as of December 31, 2018	15.9		$	1,024
Units granted	0.6		1,677
Units vested	(1.2	)	627
Units forfeited	(0.4	)	1,023
Outstanding as of March 31, 2019	14.9		$	1,080

Scheduled vesting for outstanding restricted stock units as of March 31, 2019, is as follows (in millions):



	Nine Months Ended December 31,	Year Ended December 31,
	2019	2020	2021	2022	2023	Thereafter	Total
Scheduled vesting—restricted stock units	5.6	5.5	2.5	1.0	0.2	0.1	14.9

As of March 31, 2019, there was $6.1 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, 2018 and March 31, 2019 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.

Changes in Stockholders’ Equity

The following table shows the changes in stockholders’ equity (in millions):



	Three Months Ended March 31,
	2018			2019
Total beginning stockholders' equity	$	27,709		$	43,549

Beginning and ending common stock	5			5

Beginning and ending treasury stock	(1,837		)	(1,837		)

Beginning additional paid-in capital	21,389			26,791
Stock-based compensation and issuance of employee benefit plan stock	1,174			1,268
Ending additional paid-in capital	22,563			28,059

Beginning accumulated other comprehensive loss	(484		)	(1,035		)
Other comprehensive income (loss)	17			25
Ending accumulated other comprehensive loss	(467		)	(1,010		)

Beginning retained earnings	8,636			19,625
Cumulative effect of changes in accounting principles (1)	934			7
Net income	1,629			3,561
Ending retained earnings	11,199			23,193

Total ending stockholders’ equity	$	31,463		$	48,410

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(1)	We recorded cumulative effect adjustments related to the new revenue and income tax standards in Q1 2018 and the new lease standard in Q1 2019. See Item 1 of Part I, “Financial Statements — Note 1 — Accounting Policies — Accounting Pronouncements Recently Adopted” for additional information.