Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities
Net income (loss)	$ (43,092)	$ 64,012
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	2,816	2,003
Accretion of discount on marketable securities	(838)	(1,876)
Stock-based compensation	14,743	15,159
Abandonment of capitalized intangible assets	167	90
Equity received in connection with license agreement	(4,589)
Change in fair value of equity security	1,978
Loss on disposal of assets	5	5
Gain on sale of marketable securities available-for-sale	(153)
Changes in operating assets and liabilities:
Accounts receivable	12,649	122
Interest receivable	914	(154)
Prepaid expenses and other assets	(843)	(932)
Accounts payable	324	5,197
Accrued expenses	162	(3,436)
Income taxes	91	1,604
Lease liabilities and right of use (ROU) assets	(110)	(65)
Deferred revenue	(2,446)	8,699
Net cash provided by (used in) operating activities	(18,222)	90,428
Cash flows from investing activities
Purchase of marketable securities	(302,319)	(244,499)
Purchase of intangible assets	(1,467)	(2,164)
Purchase of property and equipment	(2,695)	(1,867)
Proceeds from maturities of marketable securities	378,454	159,773
Net cash provided by (used in) investing activities	71,973	(88,757)
Cash flows from financing activities
Proceeds from issuance of common stock upon exercise of stock awards	4,746	3,906
Proceeds from issuance of common stock under the Employee Stock Purchase Plan	725	734
Net cash provided by financing activities	5,471	4,640
Net increase in cash and cash equivalents	59,222	6,311
Cash and cash equivalents, beginning of period	50,312	26,246
Cash and cash equivalents, end of period	109,534	32,557
Cash paid during the period for:
Interest	11	7
Income taxes		150
Supplemental disclosures of non-cash investing activities
Net unrealized gain on marketable securities	$ 322	$ 2,600