Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (91,298)	$ (64,339)	$ (112,387)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	2,678	2,639	1,295
Amortization of deferred financing costs	236	375	346
Amortization of deferred compensation for services	253	192	194
Amortization of premium on investment securities	(1,136)	11	(86)
Stock-based compensation	9,226	9,590	5,317
Change in derivative liabilities	111
Non-cash interest associated with debt discount accretion	303
Changes in operating assets and liabilities:
Accounts receivable	10,711	(10,796)
Prepaid expenses and other assets	525	(2,148)	1,562
Accounts payable	(2,730)	(1,027)	2,148
Accrued compensation and benefits	(506)	68	795
Accrued and other liabilities	1,353	245	8,282
Net cash used in operating activities	(70,274)	(65,190)	(92,534)
Investing activities
Proceeds from maturities of investments	138,600	133,701	7,600
Sales and redemptions of investments	850	17,957
Purchases of investments	(169,033)	(84,013)	(133,810)
Purchases of property and equipment	(311)	(2,355)	(4,866)
Net cash (used in) provided by investing activities	(29,894)	65,290	(131,076)
Financing activities
Proceeds from loan payable, net of issuance costs	49,292
Proceeds from issuance of common stock, net of issuance costs	53,770		190,573
Proceeds from issuance of common stock under stock plans	491	685	631
Net cash provided by financing activities	103,553	685	191,204
Net decrease in cash and cash equivalents	3,385	785	(32,406)
Cash and cash equivalents at beginning of period	75,383	74,598	107,004
Cash and cash equivalents at end of period	78,768	75,383	74,598
Supplementary disclosure of cash flow information:
Income taxes paid	4	3
Supplementary disclosure of non-cash financing information:
Issuance of derivative in connection with issuance of loan payable	546
Acquisition of property and equipment included in accounts payable and accrued liabilities		55	730
Services settled through the issuance of common stock	$ 303	$ 201	$ 187