Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2018

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

Stock-Based Compensation Expense

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

	Year Ended December 31,
	2016		2017		2018
Cost of sales	$	16	$	47	$	73
Fulfillment	657		911		1,121
Marketing	323		511		769
Technology and content	1,664		2,305		2,888
General and administrative	315		441		567
Total stock-based compensation expense (1)	$	2,975	$	4,215	$	5,418

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(1) The related tax benefits were $907 million, $860 million, and $1.1 billion for 2016, 2017, and 2018. In 2017 and 2018, the tax benefit reflects the permanent reduction in the U.S. statutory corporate tax rate from 35% to 21%.

Summary of Restricted Stock Unit Activity

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, 2016	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
Units granted	5.0		1,522
Units vested	(7.1	)	578
Units forfeited	(2.1	)	862
Outstanding as of December 31, 2018	15.9		$	1,024

Scheduled Vesting of Outstanding Restricted Stock Units

Scheduled vesting for outstanding restricted stock units as of December 31, 2018, is as follows (in millions):

	Year Ended
	2019	2020	2021	2022	2023	Thereafter	Total
Scheduled vesting — restricted stock units	6.9	5.6	2.4	0.8	0.1	0.1	15.9