Common Stock and Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2017

Common Stock and Stockholders'Equity

Summary of activity under the Plan

Activity under the Plan is as follows (in thousands, except share and per share amounts and years):

					Weighted-
			Weighted-		Average
	Shares	Number of	Average		Remaining	Aggregate
	Available	Shares	Exercise		Contractual	Intrinsic
	for Grant	Outstanding	Price		Life in Years	Value
Balance at December 31, 2016	343,216	6,839,868	$	11.70	8.64	$	36,795
Increase in Plan authorized shares	3,676,722	—	$	—	—	$	—
Restricted Stock Units granted	(144,618)	—	$	—	—	$	—
Stock option grants	(2,979,721)	2,979,721	$	22.52	—	$	—
Stock options exercised	—	(479,339)	$	9.01	—	$	8,810
Stock options forfeited	519,361	(519,361)	$	16.25	—	$	3,377
Balance at June 30, 2017	1,414,960	8,820,889	$	15.24	8.65	$	171,676
Vested or expected to vest June 30, 2017		8,820,889	$	15.24	8.65	$	171,676
Exercisable as of June 30, 2017		2,166,249	$	8.13	7.33	$	57,552

Assumptions used for estimate of fair value of options

	Six Months Ended June 30,
	2017		2016
Volatility		45.8% – 47.7%		44.7% – 46.0%
Expected life (in years)		6.1		6.0
Risk-free interest rate		1.81% - 2.30%		1.24% - 1.91%
Dividend yield		–		–
Weighted-average fair value of underlying common stock	$	22.52	$	5.71

Schedule of activity under the RSU's

Activity under the RSU’s is as follows (in thousands, except share and per share amounts and years):

		Weighted-Average
		Grant Date
	Shares	Fair Value Per Share
Outstanding at beginning of period	—	$	—
Granted	144,618	$	30.63
Outstanding at June 30, 2017	144,618	$	30.63
Vested and deferred at June 30, 2017	—	$	—
Non-vested at June 30, 2017	144,618	$	30.63