Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating Activities:
Net loss	$ (42,203)	$ (20,187)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	715	588
Amortization of discount on marketable securities	(1,238)	(459)
Gain on free shares	(545)
Stock-based compensation	4,523	3,050
Build-to-suit leases		217
Net loss on lease termination	218
Other	44
Net changes in operating assets and liabilities:
Interest receivable	(219)	(133)
Accounts receivable	(2,638)	(232)
Prepaid expenses and other assets	1,152	(1,334)
Operating lease right-of-use assets	782
Prepaid rent	(12,101)
Accounts payable and accrued liabilities	10,183	(77)
Accrued compensation and employee benefits	(3,306)	(2,268)
Deferred revenues	(370)	2,701
Long-term portion of lease liabilities	(66)
Other non-current liabilities	(50)
Net cash used in operating activities	(45,119)	(18,134)
Investing Activities:
Purchases of marketable securities	(87,075)	(63,401)
Maturities of marketable securities	78,253	54,450
Purchases of property and equipment	(5,977)	(2,616)
Net cash used in investing activities	(14,799)	(11,567)
Financing Activities:
Taxes paid related to net share settlement of equity awards	(265)	(14)
Proceeds from issuance of common stock	481	10,597
Net cash provided by financing activities	216	10,583
Effects of changes in foreign exchange rates	252
Net increase in cash, cash equivalents, and restricted cash	(59,450)	(19,118)
Cash, cash equivalents, and restricted cash, beginning of period	143,918	53,326
Cash, cash equivalents, and restricted cash, end of period	84,468	34,208
Supplemental disclosure of non-cash activities:
Right-of-use assets obtained in exchange for lease obligations	6,676
Property and Equipment [Member]
Supplemental disclosure of non-cash activities:
Property and equipment included in accrued liabilities	$ 1,834	$ 1,227