Stock-Based Compensation and Warrants (Tables)

6 Months Ended

Jun. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Activity Under Stock Incentive Plans

A summary of activity under the stock incentive plans is as follows (in thousands, except share data and price per share):

	Shares Subject to Outstanding Options			Weighted- Average Exercise Price Per Share		Aggregate Intrinsic Value
Outstanding — Balance at December 31, 2017		288,129		$	63.62	$	—
Options granted		44,910			18.27
Options exercised		(44,354	)		21.73
Options cancelled and forfeited		(5,426	)		32.21
Outstanding — Balance at June 30, 2018		283,259		$	63.69	$	6
Vested and expected to vest — June 30, 2018		277,258		$	64.20	$	5
Exercisable —June 30, 2018		132,977		$	89.94	$	—

Weighted-Average Fair Value Assumption of Stock Option Awards

The fair value of stock option awards to executives, employees, and other service providers was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumption. 

	Three Months Ended June 30,				Six Months Ended June 30,
		2018		2017		2018		2017
Expected term (years)		9.06	5.50 - 6.26			8.43	5.50 - 6.26
Expected volatility	92%		80 - 82%		94%		80 - 82%
Risk-free interest rate	2.95%		1.80 - 1.91%		2.90%		1.80 - 1.91%
Dividend yield		—		—		—		—