Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Operating activities:
Net loss	$ (68,438)	$ (52,142)
Adjustments to reconcile net loss to net cash used in operating activities:
License fee obligation	12,846
Loss on debt extinguishment		696
Stock-based compensation	4,221	3,399
Depreciation and amortization of property and equipment	1,600	1,893
Amortization of premium on investment securities, net of accretion of discounts	97	465
Amortization of non-cash financing costs	321	260
Other	2,071	(671)
Increase (decrease) in cash resulting from changes in:
Other current and long-term assets	376	(1,788)
Accounts payable, accrued expenses and other liabilities	(641)	(2,342)
Net cash used in operating activities	(47,547)	(50,230)
Investing activities:
Purchases of investment securities	(67,937)	(99,782)
Maturities and sales of investment securities	68,260	89,523
Purchases of property and equipment	(206)	(1,056)
Proceeds from sale of property and equipment	37
Net cash provided by (used in) investing activities	154	(11,315)
Financing activities:
Proceeds from issuance of common stock, net of issuance costs	82,782	53,892
Proceeds from borrowings under term loan facility		17,116
Repayments of borrowings under term loan facility		(17,438)
Proceeds from exercise of stock options	1,512	399
Repayments under other long-term obligations	(94)	(89)
Net cash provided by financing activities	84,200	53,880
Net change in cash and cash equivalents	36,807	(7,665)
Cash and cash equivalents at beginning of period	24,340	46,383
Cash and cash equivalents at end of period	61,147	38,718
Supplemental disclosures of cash flow information:
Cash paid for interest	1,317	1,551
Noncash investing and financing activities:
Final payment obligation to Lenders recorded as debt discount (see note 6)		1,600
Unrealized gain on investment securities	$ 31	312
Amounts capitalized under build-to-suit lease transaction		5,733
Interest capitalized during construction period for build-to-suit lease transaction		$ 642