Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (81,836,000)	$ (55,748,000)	$ (56,680,000)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	2,276,000	1,591,000	1,490,000
Loss from valuation of convertible note payable	0	1,811,000	12,118,000
Stock-based compensation	12,872,000	15,368,000	7,039,000
Amortization of premium on investments, net	666,000	1,472,000	1,597,000
Other	1,043,000	98,000	18,000
Change in operating assets and liabilities:
Contracts and other receivables	8,364,000	(9,746,000)	(196,000)
Prepaid materials	(616,000)	(3,460,000)	(641,000)
Prepaid expenses and other assets	(778,000)	(683,000)	(1,241,000)
Accounts payable	3,123,000	529,000	940,000
Accrued liabilities	(871,000)	2,094,000	556,000
Accrued compensation	(74,000)	284,000	811,000
Deferred revenue	(1,194,000)	(3,090,000)	(5,039,000)
Deferred rent and other liabilities	143,000	(379,000)	(282,000)
Net cash used in operating activities	(56,882,000)	(49,859,000)	(39,510,000)
Investing activities
Purchases of short-term investments	(65,110,000)	(78,401,000)	(113,417,000)
Sales and maturities of short-term investments	101,387,000	101,306,000	85,421,000
Purchases of property and equipment	(913,000)	(1,363,000)	(1,146,000)
Acquisition of intangibles	(53,000)	(67,000)	(65,000)
Net cash provided by (used in) investing activities	35,311,000	21,475,000	(29,207,000)
Financing activities
Proceeds from borrowing under term loan, net	19,768,000	0	0
Proceeds from issuance of common stock, net	641,000	492,000	86,146,000
Proceeds from exercise of common stock options	310,000	6,680,000	2,233,000
Principal payments on other long-term obligations	(167,000)	(155,000)	(142,000)
Net cash provided by financing activities	20,552,000	7,017,000	88,237,000
Net (decrease) increase in cash and cash equivalents	(1,019,000)	(21,367,000)	19,520,000
Cash and cash equivalents at beginning of period	15,960,000	37,327,000	17,807,000
Cash and cash equivalents at end of period	14,941,000	15,960,000	37,327,000
Supplemental disclosure of cash flow information
Net changes in restricted cash	(1,256,000)	1,256,000	0
Interest paid	(981,000)	(27,000)	(39,000)
Income taxes paid	(1,000)	(1,000)	(1,000)
Supplemental disclosure of non-cash investing and financing activities
Allowance for tenant improvements	6,653,000	1,656,000	0
Amounts accrued for property and equipment	292,000	223,000	76,000
Amounts accrued for patent expenditures	$ 0	$ 28,000	$ 42,000