Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities
Net loss	$ (21,982)	$ (47,412)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	505	353
Non-cash stock-based compensation	7,484	4,756
Loss on disposal of fixed assets	2
Premium on purchases of marketable securities	(2,058)	(919)
Amortization of premium on marketable securities	1,433	1,610
Changes in operating assets and liabilities:
Accounts and other receivables	(283)	438
Prepaid expenses and other assets	(257)	436
Accounts payable	(1,504)	(1,955)
Accrued expenses	6,263	3,349
Deferred revenue	31,592
Net cash provided by (used in) operating activities	21,195	(39,344)
Cash flows from investing activities
Purchases of fixed assets	(611)	(317)
Purchases of marketable securities	(471,726)	(68,803)
Maturities of marketable securities	227,129	105,622
Net cash provided by (used in) provided by investing activities	(245,208)	36,502
Cash flows from financing activities
Proceeds from issuance of common stock	297,137
Proceeds from exercise of stock options	3,473	3,967
Proceeds from sale of common stock under Employee Stock Purchase Plan	137	96
Proceeds from exercise of warrants		5,251
Payment of deferred financing costs		(82)
Borrowings of debt	229
Repayments of debt	(196)	(237)
Net cash provided by financing activities	300,780	8,995
Net increase in cash and cash equivalents	76,767	6,153
Cash and cash equivalents, beginning of period	73,664	33,457
Cash and cash equivalents, end of period	150,431	39,610
Supplemental disclosure of cash flow information
Cash paid for interest	39	25
Supplemental disclosure of non-cash financing and investing activities
Cashless exercise of warrants	53	2,848
Purchases of equipment in accounts payable or accrued expenses	$ 49	$ 352