Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating activities
Net loss	$ (83,597)	$ (61,389)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,913	791
Noncash interest expense	8	19
Stock-based compensation	13,311	5,219
Accretion of premiums and discounts on investments	(1,675)	76
Changes in assets and liabilities:
Unbilled accounts receivable		502
Prepaid expenses and other current assets	3,088	(2,608)
Other assets	(3,584)	4
Accounts payable	(785)	297
Accrued expenses	11,595	3,953
Deferred revenue	7,607	(5,830)
Deferred rent	(303)	369
Net cash used in operating activities	(52,422)	(58,597)
Investing activities
Purchases of property and equipment	(9,634)	(457)
Purchases of investments	(489,285)	(209,892)
Maturities of investments	265,000	96,000
Net cash used in by investing activities	(233,919)	(114,349)
Financing activities
Principal payments on loan payable	(833)	(1,417)
Principal payments on capital lease obligations	(39)
Payment of offering costs	(281)	(716)
Proceeds from public offering of common stock, net of commissions and underwriting discounts		216,200
Proceeds from issuance of common stock, net of repurchases	4,730	1,656
Net cash provided by financing activities	3,577	215,723
Net (decrease) increase in cash, cash equivalents, and restricted cash	(282,764)	42,777
Cash, cash equivalents, and restricted cash at beginning of period	405,072	53,336
Cash, cash equivalents, and restricted cash at end of period	122,308	96,113
Supplemental cash flow information
Public offering costs incurred but unpaid at period end		306
Property and equipment purchases unpaid at period end	2,114	1,920
Cash paid for interest	$ 31	$ 91