Stockholders' Equity (Stock Option and RSU Activity) (Detail) shares in Thousands	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Number of options
Number of options, beginning outstanding number (in shares) | shares	10,649
Number of options, Granted (in shares) | shares	7,165
Number of options, Exercised (in shares) | shares	(4)
Number of options, Canceled/forfeited/expired (in shares) | shares	(2,995)
Number of options, ending outstanding number (in shares) | shares	14,815
Weighted average exercise price
Weighted average exercise price, beginning balance (in dollars per share) | $ / shares	$ 4.44
Weighted average exercise price, Granted (in dollars per share) | $ / shares	0.98
Weighted average exercise price, Exercised (in dollars per share) | $ / shares	0.38
Weighted average exercise price, Canceled/forfeited/expired (in dollars per share) | $ / shares	5.99
Weighted average exercise price, ending balance (in dollars per share) | $ / shares	$ 2.45
Restricted Stock Units (RSUs)
Number of RSUs
Shares outstanding (in shares) | shares	416
Shares granted (in shares) | shares	68
Shares vested in period (in shares) | shares	(209)
Shares canceled in period (in shares) | shares	(54)
Shares outstanding (in shares) | shares	221
Weighted average grant date fair value
Beginning balance (usd per share) | $ / shares	$ 0.89
Granted (usd per share) | $ / shares	0.65
Exercised (usd per share) | $ / shares	0.87
Canceled/forfeited/expired (usd per share) | $ / shares	0.89
Ending balance (usd per share) | $ / shares	$ 0.84