Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net income (loss)	$ (68,269)	$ 95,631
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Stock-based compensation expense	19,979	11,755
Net amortization of premiums and accretion of discounts on marketable debt securities	(1,085)	889
Depreciation and amortization	5,229	2,671
Net realized losses (gains) on sales and maturities of marketable securities	(20)	15
Imputed interest income from licensing	(2,091)	(8,362)
Unrealized gains on marketable equity securities	(29,420)
Other non-cash adjustments	357	12
Changes in operating assets and liabilities
Accounts receivable	(10,699)	6,986
Prepaid expenses	(1,418)	(974)
Other current assets	1,286	(3,072)
Operating lease right-of-use assets	1,712
Other assets	(2,150)	(788)
Accounts payable	5,339	156
Accrued expenses and other current liabilities	1,990	3,438
Deferred revenue	(600)	600
Operating lease liabilities	(1,832)
Deferred rent		(41)
Other liabilities	(507)	(128)
Net cash provided by (used in) operating activities	(82,199)	108,788
Cash flows from investing activities
Purchases of marketable securities	(127,925)	(319,897)
Maturities of marketable securities	218,019	116,110
Purchases of property and equipment	(10,689)	(8,389)
Net cash provided by (used in) investing activities	79,405	(212,176)
Cash flows from financing activities
Proceeds from exercise of stock options	5,513	9,051
Proceeds from issuance of common stock under employee stock purchase plan	1,314	850
Proceeds from public offerings of common stock, net of underwriting discounts and commissions		189,716
Issuance costs for public offerings of common stock		(462)
Net cash provided by financing activities	6,827	199,155
Net increase in cash and cash equivalents and restricted cash	4,033	95,767
Cash and cash equivalents and restricted cash
Beginning of period	76,614	46,881
End of period	$ 80,647	142,648
Supplemental disclosures of non-cash investing and financing activities
Purchases of property and equipment in accounts payable and accrued expenses		174
Issuance costs for public offerings of common stock in accounts payable and accrued expenses		$ 157