CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated deficit
Beginning balance at Dec. 31, 2015	$ 48,597	$ 4	$ 353,233	$ 29	$ (304,669)
Beginning Balance, Shares at Dec. 31, 2015		42,551
Issuance of stock in lieu of accrued bonuses	364	$ 0	364	0	0
Issuance of stock in lieu of accrued bonuses, Shares		28
Stock-based compensation expense	9,236	$ 0	9,236	0	0
Employee stock purchase plans, purchases	921	$ 0	921	0	0
Employee stock purchase plans, shares		138
Restricted stock awards issued, net of cancellations	0	$ 0	0	0	0
Restricted stock awards issued, net of cancellations, Shares		421
Shares issued under stock-based compensation plans, net of cancellations	712	$ 0	712	0	0
Shares issued under stock-based compensation plans, net of cancellations, Shares		99
Net loss	(50,601)	$ 0	0	0	(50,601)
Temporary Equity, Foreign Currency Translation Adjustments	77	0	0	77	0
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	77
Net unrealized gains (losses) on marketable securities, net of tax	(11)	0	0	(11)	0
Ending Balance at Dec. 31, 2016	38,151	$ 4	393,322	95	(355,270)
Ending Balance, Shares at Dec. 31, 2016		46,554
Stock-based compensation expense	12,170	$ 0	12,170	0	0
Employee stock purchase plans, purchases	1,016	$ 0	1,016	0	0
Employee stock purchase plans, shares		175
Restricted stock awards issued, net of cancellations	0	$ 0	0	0	0
Restricted stock awards issued, net of cancellations, Shares		955
Shares issued under stock-based compensation plans, net of cancellations	287	$ 0	287	0	0
Shares issued under stock-based compensation plans, net of cancellations, Shares		42
Issuance of common stock, net of offering expenses	80,732	$ 2	80,730	0	0
Issuance of common stock, net of offering expenses, Shares		7,340
Net loss	(61,850)	$ 0	0	0	(61,850)
Temporary Equity, Foreign Currency Translation Adjustments	(84)	0	0		0
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	(84)
Net unrealized gains (losses) on marketable securities, net of tax	(2)	0	0	(2)	0
Ending Balance at Dec. 31, 2017	71,157	$ 6	487,525	9	(416,383)
Ending Balance, Shares at Dec. 31, 2017		55,066
Stock-based compensation expense	11,697	$ 0	11,697	0	0
Employee stock purchase plans, purchases	1,061	$ 0	1,061	0	0
Employee stock purchase plans, shares		253
Restricted stock awards issued, net of cancellations	0	$ 0	0	0	0
Restricted stock awards issued, net of cancellations, Shares		916
Shares issued under stock-based compensation plans, net of cancellations	21	$ 0	21	0	0
Shares issued under stock-based compensation plans, net of cancellations, Shares		5
Net loss	(50,500)	$ 0	0	0	(50,500)
Temporary Equity, Foreign Currency Translation Adjustments	44	0	0		0
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	44
Net unrealized gains (losses) on marketable securities, net of tax	27	0	0	27	0
Ending Balance at Dec. 31, 2018	$ 33,547	$ 6	$ 500,344	$ 80	$ (466,883)
Ending Balance, Shares at Dec. 31, 2018		56,240