Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities
Net loss	$ (55,361)	$ (23,200)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	1,552	864
Amortization of premium on marketable securities	406	1,356
Stock-based compensation	9,353	6,607
Abandonment of capitalized intangible assets	50	225
Changes in operating assets and liabilities:
Accounts receivable	(140)	(6,260)
Interest receivable	(595)	(158)
Prepaid expenses and other assets	(6,468)	(4,727)
Accounts payable	(1,953)	3,500
Accrued expenses	443	(2,708)
Income taxes	(157)	390
Deferred rent	206	(12)
Deferred revenue		701
Net cash used in operating activities	(52,664)	(23,422)
Cash flows from investing activities
Purchase of marketable securities	(285,809)	(33,440)
Purchase of intangible assets	(882)	(1,092)
Purchase of property and equipment	(3,416)	(1,256)
Proceeds from sale and maturities of marketable securities	112,526	53,866
Loan receivable	86	(86)
Net cash provided by (used in) investing activities	(177,495)	17,992
Cash flows from financing activities
Proceeds from issuance of common stock upon exercise of stock awards	2,915	1,680
Proceeds from issuance of common stock under the Employee Stock Purchase Plan	504	442
Proceeds from issuance of common stock	260,245
Common stock issuance cost	(14,741)
Net cash provided by financing activities	248,923	2,122
Net increase (decrease) in cash and cash equivalents	18,764	(3,308)
Cash and cash equivalents, beginning of period	16,528	14,528
Cash and cash equivalents, end of period	35,292	11,220
Cash paid during the period for:
Interest	6	5
Taxes	170	60
Supplemental disclosures of non-cash investing activities
Unrealized gain on marketable securities, net of tax	$ 200	$ 201