Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Stock-Based Compensation Expense Related to Grants of Employee Stock Options, Restricted Stock Units (ZSUs) and Performance-Based Awards

We recorded stock-based compensation expense related to grants of employee stock options, ZSUs and performance-based awards in our consolidated statements of operations as follows (in thousands):

	Year Ended December 31,
	2017		2016		2015
Cost of revenue	$	1,838	$	3,720	$	4,547
Research and development		42,176		84,236		94,548
Sales and marketing		7,281		7,254		7,501
General and administrative		13,220		12,251		24,979
Total stock-based compensation expense	$	64,515	$	107,461	$	131,575

Weighted-Average Grant Date Fair Value of Stock Options and Related Assumptions

The following table presents the weighted-average grant date fair value of stock options and related assumptions used to estimate the fair value in our consolidated financial statements: 

	Year Ended December 31,
	2017			2016			2015
Expected term, in years		6			6			6
Risk-free interest rates		2.12	%		1.53	%		1.65	%
Expected volatility		46	%		50	%		53	%
Dividend yield		—			—			—
Weighted-average estimated fair value of stock options granted    during the year	$	1.75		$	1.30		$	1.51

Schedule of Share Based Compensation Stock Option Activity

The following table shows stock option activity for 2017 (in thousands, except weighted-average exercise price and remaining contractual term):

	Outstanding Options
	Stock Options			Weighted- Average Exercise Price		Aggregate Intrinsic Value of Stock Options Outstanding		Weighted- Average Contractual Term (in years)
Balance as of December 31, 2016		36,858		$	2.08	$	26,411		6.81
Granted		1,681			3.76
Forfeited, expired and cancelled		(3,084	)		3.63
Exercised		(2,491	)		1.33
Balance as of December 31, 2017		32,964		$	2.07	$	64,114		6.32
As of December 31, 2017
Exercisable options		16,880		$	1.43	$	43,878		4.11
Vested and expected to vest		32,964		$	2.07	$	64,114		6.32

Schedule of Share Based Compensation Restricted Stock Units Award Activity

The following table shows a summary of ZSU activity for 2017 (in thousands, except weighted-average fair value): 

	Outstanding ZSUs
				Weighted-
				Average Grant Date		Aggregate
				Fair Value		Intrinsic Value of
	Shares			(per share)		Unvested ZSUs
Unvested as of December 31, 2016		59,452		$	2.66	$	152,792
Granted		22,210			3.52
Vested		(20,774	)		2.83
Forfeited		(15,410	)		2.67
Unvested as of December 31, 2017		45,478		$	3.00	$	181,912

Common Stock Reserved For Future Issuance

As of December 31, 2017, we had reserved shares of common stock for future issuance as follows (in thousands):

	December 31, 2017
Stock options outstanding		32,964
ZSUs outstanding		45,478
2011 Equity Incentive Plan		126,918
2011 Employee Stock Purchase Plan		92,875
Total		298,235