CONDENSED STATEMENTS OF CASH FLOWS - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash Flows from Operating Activities:
Net loss	$ (39,211,000)	$ (36,565,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-employee stock option expense		142,000
Stock-based compensation	5,127,000	4,013,000
Issuance of common stock for services rendered	129,000	90,000
Accretion of premiums and discounts on investments	466,000	397,000
Depreciation and amortization expense	484,000	346,000
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(48,000)	(1,163,000)
Accounts payable, accrued expenses, and other liabilities	937,000	(1,590,000)
Deferred revenue	(833,000)
Net cash used in operating activities	(32,949,000)	(34,330,000)
Cash Flows from Investing Activities:
Purchases of available-for-sale securities	(2,946,000)	(63,106,000)
Proceeds from maturity of available-for-sale securities	29,946,000	23,602,000
Proceeds from sale of available-for-sale securities	1,974,000	999,000
Purchases of property and equipment	(369,000)	(659,000)
Net cash provided by (used in) investing activities	28,605,000	(39,164,000)
Cash Flows from Financing Activities:
Proceeds from equity financings, net of issuance costs		80,599,000
Proceeds from exercise of common stock warrants and options and employee stock purchases	111,000	987,000
Payments on note payable	(193,000)	(59,000)
Payments on capital lease	(6,000)	(8,000)
Net cash (used in) provided by financing activities	(88,000)	81,519,000
Net increase (decrease) in cash and cash equivalents	(4,432,000)	8,025,000
Cash and cash equivalents, beginning of period	26,586,000	19,971,000
Cash and cash equivalents, end of period	$ 22,154,000	$ 27,996,000