Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating Activities:
Net loss	$ (36,827)	$ (29,123)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,207	618
Amortization of (discount) premium on marketable securities	(1,656)	32
Stock-based compensation	6,564	4,751
Other	465
Net changes in operating assets and liabilities:
Interest receivable	(322)	(32)
Accounts receivable	(1,634)	1,477
Prepaid expenses and other assets	(3,564)	(1,039)
Accounts payable and accrued liabilities	2,733	2,654
Accrued compensation and employee benefits	(1,133)	24
Deferred revenues	138,474	64,346
Net cash provided by operating activities	104,307	43,708
Investing Activities:
Purchases of marketable securities	(451,240)	(159,166)
Maturities of marketable securities	133,297	83,029
Purchases of property and equipment	(5,768)	(2,175)
Net cash used in investing activities	(323,711)	(78,312)
Financing Activities:
Proceeds from public offering of common stock, net of issuance costs	215,756	81,562
Taxes paid related to net share settlement of equity awards	(57)	(7)
Proceeds from issuance of common stock	13,285	825
Net cash provided by financing activities	228,984	82,380
Net increase in cash, cash equivalents, and restricted cash	9,580	47,776
Cash, cash equivalents, and restricted cash, beginning of period	53,326	22,061
Cash, cash equivalents, and restricted cash, end of period	62,906	69,837
Property and Equipment [Member]
Supplemental disclosure of noncash investing activities:
Property and equipment included in accrued liabilities	$ 1,836	$ 226