Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities
Net loss	$ (160,399)	$ (97,869)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	942	1,142
Amortization of premium on investment securities	2,548	2,669
Stock-based compensation expense	16,032	6,876
Gain on retirement of equipment	(11)
Unrealized loss on foreign currency forward contracts		114
Changes in operating assets and liabilities:
Receivables from collaborators	57	236
Prepaid expenses and other current assets	(14,753)	(797)
Prepaid and other long-term assets	2,625	(9,954)
Accounts payable	(5,332)	9,354
Accrued compensation and employee benefits	383	(40)
Accrued research and development	11,197	(1,973)
Accrued and other liabilities	2,276	(610)
Deferred revenue	(7,154)	33,787
Other long-term liabilities	1,522	418
Net cash used in operating activities	(150,067)	(56,647)
Investing activities
Purchases of property and equipment	(3,150)	(1,329)
Proceeds from sale of equipment	11
Purchases of investments	(187,166)	(156,009)
Proceeds from maturities of investments	241,422	145,661
Net cash provided by/(used) in investing activities	51,117	(11,677)
Financing activities
Proceeds from public offering of common stock, net of underwriters discount	108,772
Payment of public offering cost	(358)	(239)
Proceeds from issuance of common stock pursuant to equity award plans	10,744	3,934
Net cash provided by financing activities	119,158	3,695
Net increase/(decrease) in cash and cash equivalents	20,208	(64,629)
Cash and cash equivalents at beginning of period	57,514	117,773
Cash and cash equivalents at end of period	$ 77,722	$ 53,144