Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2018

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

Estimated Fair Value of Certain Stock-Based Awards Using Black-Scholes-Merton Valuation Model

The Company estimated the fair value of certain stock-based awards using a Black-Scholes-Merton valuation model with the following assumptions: 

	Years ended December 31,
Stock options	2018	2017	2016
Weighted-average expected term (years)	6.02	5.99	5.75
Weighted-average volatility	65%	65%	65%
Risk-free interest rate	2.27%-2.62%	2.02%-2.08%	1.01%-1.76%
Expected dividends	—  %	—  %	—  %
Stock appreciation units
Weighted-average expected term (years)	1.94	2.30	2.77
Weighted-average volatility	66%	69%	61%
Risk-free interest rate	1.03%-2.81%	0.51%-1.62%	0.45%-1.47%
Expected dividends	0	—  %	—  %
ESPP
Weighted-average expected term (years)	0.72	0.72	0.73
Weighted-average volatility	61%	61%	54%
Risk-free interest rate	1.93%-2.59%	0.91%-1.31%	0.39%-0.45%
Expected dividends	—  %	—  %	—  %

Summary of Stock Based Compensation Expense

Unamortized stock-based compensation costs capitalized as part of inventory were immaterial in each of the periods presented (in thousands):

	Years ended December 31,
	2018		2017		2016
Cost of goods sold	$	2,596	$	1,098	$	3,130
Research and development	3,570		2,491		4,760
Sales and marketing	3,248		1,697		4,105
General and administrative	4,728		2,920		5,081
	$	14,142	$	8,206	$	17,076

Summary of Stock Option and Restricted Stock Unit Activity

The following table summarizes the Company’s stock option and restricted stock unit, or RSU, activity during the year ended December 31, 2018: 

			Stock Options				Restricted Stock Units
	Shares Available for Grant		Number of Shares		Weighted Average Exercise Price		Number of Units		Weighted Average Grant Date Fair Value
Balance at December 31, 2017	959,136		3,933,529		$	5.55	2,404,637		$	9.02
Authorized for issuance	1,947,667		—		—		—		—
Granted	(2,300,056	)	158,116		6.64		1,446,940		6.83
Exercised/Converted	—		(778,811	)	4.39		(1,123,893	)	8.82
Cancelled/Forfeited	495,576		(110,089	)	10.07		(241,656	)	8.74
Balance at December 31, 2018	1,102,323		3,202,745		$	5.73	2,486,028		$	7.87

Summary of Information about Stock Options Outstanding

The following table summarizes information about stock options outstanding as of December 31, 2018: 

	Options Outstanding
	Number of Shares	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in Thousands)
Vested and expected to vest	3,152,837	$	5.70	5.31	$	4,936,208
Exercisable	2,701,433	$	5.32	4.78	$	4,864,959

Summary of Information about Restricted Stock Units Outstanding

The following table summarizes information about RSUs outstanding as of December 31, 2018:

	Restricted Stock Units Outstanding
	Number of Shares	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in Thousands)
Vested and expected to vest	2,166,565	$	—	1.26	$	14,039,340

Stock Options - Schedule of Share-based Payment Award, Equity Instruments Other Than Options, Valuation Assumptions

The fair value of market-based RSUs was measured on the grant date using Monte Carlo simulation model with the following assumptions:

	Years ended December 31,
	2018	2017
Weighted-average volatility	66%	—%
Risk-free interest rate	2.79%	—%
Expected dividends	—%	—%

Summary of Stock Appreciation Unit Activity

The following table summarizes the Company’s stock appreciation unit activity during the year ended December 31, 2018: 

	Stock Appreciation Units		Weighted-Average Exercise Price
Stock appreciation units outstanding as of December 31, 2017	239,824		$	4.95
Stock appreciation units exercised	(31,272	)	$	4.32
Stock appreciation units cancelled	(15,680	)	$	6.72
Stock appreciation units outstanding as of December 31, 2018	192,872		$	4.91