Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (94,940)	$ (91,298)	$ (64,339)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	2,501	2,678	2,639
Amortization of deferred financing costs	670	236	375
Amortization of deferred compensation for services	309	253	192
Amortization of (premium) discount on investment securities	(698)	(1,136)	11
Non-cash lease expense	1,839
Stock-based compensation	9,936	9,226	9,590
Change in derivative liabilities	436	111
Non-cash interest associated with debt discount accretion	478	303
Changes in operating assets and liabilities:
Unbilled revenue	4,250
Accounts receivable	85	10,711	(10,796)
Prepaid expenses and other assets	93	525	(2,148)
Accounts payable	39	(2,730)	(1,027)
Accrued compensation and benefits	1,730	(506)	68
Lease liabilities	(1,892)
Accrued and other liabilities	(5,861)	1,353	245
Deferred revenue	4,541
Net cash used in operating activities	(76,484)	(70,274)	(65,190)
Investing activities
Proceeds from maturities of investments	124,369	138,600	133,701
Sales and redemptions of investments	2,000	850	17,957
Purchases of investments	(102,671)	(169,033)	(84,013)
Purchases of property and equipment	(325)	(311)	(2,355)
Net cash provided by (used in) investing activities	23,373	(29,894)	65,290
Financing activities
Proceeds from underwritten public offering, net of issuance costs	134,927	53,770
Proceeds from issuance of common stock upon private placement, net of issuance costs	19,975
Proceeds from issuance of common stock under stock plans	396	491	623
Issuance of common stock upon exercise of options	178		62
Proceeds from loan payable, net of issuance costs		49,292
Net cash provided by financing activities	155,476	103,553	685
Net increase in cash and cash equivalents	102,365	3,385	785
Cash and cash equivalents at beginning of period	78,768	75,383	74,598
Cash and cash equivalents at end of period	181,133	78,768	75,383
Supplementary disclosure of cash flow information:
Income taxes paid	2	4	3
Supplementary disclosure of non-cash activities:
Right-of-use assets obtained in exchange for lease obligations	5,810
Issuance of common stock for services	$ 312	303	201
Issuance of derivative in connection with issuance of loan payable		$ 546
Acquisition of property and equipment included in accounts payable and accrued liabilities			$ 55