Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities:
Net loss	$ (68,889)	$ (54,568)	$ (71,658)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	2,359	1,498	997
Amortization of (discount) premium on marketable securities	(5,829)	(673)	221
Foreign currency transaction losses, net	602
Net loss on disposal of property and equipment		12
Stock-based compensation	14,677	9,089	15,057
Benefit from income taxes			(14)
Build-to-suit leases	966	80	99
Net changes in operating assets and liabilities:
Interest receivable	(135)	(16)	83
Accounts receivable	(1,330)	1,629	(2,144)
Prepaid expenses and other assets	(2,828)	(669)	(1,112)
Accounts payable and accrued liabilities	(6,372)	3,219	(2,335)
Accrued compensation and employee benefits	2,604	2,594	137
Deferred revenues	99,364	48,984	(5,214)
Non-current liabilities	1,963
Net cash provided by (used in) operating activities	37,152	11,179	(65,883)
Investing Activities:
Acquisition of TxCell, net of cash acquired	(75,647)
Purchases of marketable securities	(451,239)	(252,328)	(218,640)
Maturities of marketable securities	391,845	178,675	237,497
Purchases of property and equipment	(43,065)	(3,751)	(732)
Net cash (used in) provided by investing activities	(178,106)	(77,404)	18,125
Financing Activities:
Proceeds from public offering of common stock, net of issuance costs	215,758	81,573
Taxes paid related to net share settlement of equity awards	(254)	(654)	(776)
Proceeds from issuance of common stock	16,205	16,571	1,113
Net cash provided by financing activities	231,709	97,490	337
Effects of changes in foreign exchange rates	(163)
Net increase in cash, cash equivalents, and restricted cash	90,592	31,265	(47,421)
Cash, cash equivalents, and restricted cash, beginning of period	53,326	22,061	69,482
Cash, cash equivalents, and restricted cash, end of period	143,918	53,326	22,061
Supplemental disclosure of non-cash investing activities:
Non controlling interest for acquisition of TxCell	1,313
Property and Equipment [Member]
Supplemental disclosure of non-cash investing activities:
Property and equipment/build-to-suit leases included in accrued liabilities	4,953	1,214
Build-to-Suit Leases [Member]
Supplemental disclosure of non-cash investing activities:
Property and equipment/build-to-suit leases included in accrued liabilities	$ 2,950	$ 20,793	$ 3,876