CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (70,242)	$ (44,459)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash litigation expense	10,315
Stock-based compensation expense	5,673	6,003
Depreciation expense	580	571
Loss on disposal of property and equipment	9	51
Amortization of premium on investments	(390)	(106)
Changes in operating assets and liabilities:
Litigation settlement payable	12,797
Deferred revenue	(4,635)
Prepaid expenses and other assets	312	(1,963)
Accounts payable	(1,389)	1,114
Withholding tax receivable	1,583
Accrued expenses and other liabilities	2,045	(584)
Net cash used in operating activities	(43,342)	(39,373)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(245)	(133)
Maturities of held-to-maturity investments	50,000	45,000
Purchases of held-to-maturity investments	(138,699)	(64,853)
Net cash used in investing activities	(88,944)	(19,986)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of underwriters' commissions	108,099
Proceeds from issuance of redeemable convertible preferred stock		70,000
Redeemable convertible preferred stock issuance costs		(750)
Proceeds from stock option exercises and issuances under Employee Stock Purchase Plan	1,832	215
Settlement of restricted stock for tax withholding	(35)	(11)
Net cash provided by financing activities	109,896	69,454
NET (DECREASE) INCREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH EQUIVALENTS	(22,390)	10,095
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH EQUIVALENTS — Beginning of period	69,533	21,981
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH EQUIVALENTS — End of period	47,143	32,076
NONCASH FINANCING ACTIVITIES:
Dividends on redeemable convertible preferred stock		6,733
Deemed dividend related to beneficial conversion feature of redeemable preferred stock		$ 6,144
Common stock issuance costs included in accounts payable and accrued expenses	349
NONCASH INVESTING ACTIVITIES:
Property and equipment purchases included in accounts payable	$ 44