Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities
Net income (loss)	$ (37,082)	$ 32,690
Adjustments to reconcile net income (loss) to net cash (used in) provided by operating activities
Depreciation and amortization	1,420	999
Amortization of premium on marketable securities	2,148	1,398
Stock-based compensation	10,211	5,858
Abandonment of capitalized intangible assets	273	107
Gain on disposal of assets	(2)
Gain on sale of marketable securities available-for-sale		(6)
Changes in operating assets and liabilities:
Accounts receivable	7,785	(3,044)
Interest receivable	(588)	(541)
Prepaid expenses and other assets	(3,971)	(2,288)
Accounts payable	2,722	934
Accrued expenses	(2,056)	2,902
Income taxes	(65)	400
Deferred rent	359	(62)
Deferred revenue	(8,171)	71,058
Net cash (used in) provided by operating activities	(27,017)	110,405
Cash flows from investing activities
Purchase of marketable securities	(49,203)	(202,147)
Purchase of intangible assets	(1,520)	(1,207)
Purchase of property and equipment	(3,834)	(1,233)
Proceeds from sale of property and equipment	2
Proceeds from sale and maturities of marketable securities	77,566	95,205
Net cash provided by (used in) investing activities	23,011	(109,382)
Cash flows from financing activities
Proceeds from issuance of common stock upon exercise of stock awards	2,669	948
Proceeds from issuance of common stock under the Employee Stock Purchase Plan	443	226
Net cash provided by financing activities	3,112	1,174
Net (decrease) increase in cash and cash equivalents	(894)	2,197
Cash and cash equivalents, beginning of period	14,528	12,590
Cash and cash equivalents, end of period	13,634	14,787
Cash paid during the period for:
Interest	8
Income taxes	790	760
Supplemental disclosures of non-cash investing activities
Unrealized gain on marketable securities available for sale	$ 344	$ 266