Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (64,339)	$ (112,387)	$ (29,621)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	2,639	1,295	829
Amortization of deferred financing costs	375	346	351
Amortization of deferred compensation for services	192	194	107
Amortization of premium on investment securities	11	(86)
Stock-based compensation	9,590	5,317	2,634
Loss from disposal of fixed assets			65
Changes in operating assets and liabilities:
Accounts receivable	(10,796)		2,584
Prepaid expenses and other assets	(2,148)	1,562	(4,083)
Accounts payable	(1,027)	2,148	(371)
Accrued compensation and benefits	68	795	718
Accrued and other liabilities	245	8,282	2,000
Deferred revenue			(47,053)
Net cash used in operating activities	(65,190)	(92,534)	(71,840)
Investing activities
Proceeds from maturities of investments	133,701	7,600
Sales and redemptions of investments	17,957
Purchases of investments	(84,013)	(133,810)
Purchases of property and equipment	(2,355)	(4,866)	(3,454)
Net cash provided by (used in) investing activities	65,290	(131,076)	(3,454)
Financing activities
Proceeds from issuance of common stock, net of issuance costs		190,573	74,323
Proceeds from issuance of common stock under stock plans	685	631	659
Other			30
Net cash provided by financing activities	685	191,204	75,012
Net increase (decrease) in cash and cash equivalents	785	(32,406)	(282)
Cash and cash equivalents at beginning of period	74,598	107,004	107,286
Cash and cash equivalents at end of period	75,383	74,598	107,004
Supplementary disclosure of cash flow information:
Income taxes paid	3		69
Supplementary disclosure of non-cash financing information:
Acquisition of property and equipment included in accounts payable and accrued liabilities	55	730
Services settled through the issuance of common stock	$ 201	$ 187	$ 194