Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating activities
Net loss	$ (105,242)	$ (62,076)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	629	742
Amortization of premium on investment securities	1,803	1,848
Stock-based compensation expense	9,931	4,726
Gain on retirement of equipment	(11)
Unrealized loss on foreign currency forward contracts		1
Changes in operating assets and liabilities:
Receivables from collaborators	57	127
Prepaid expenses and other current assets	(10,220)	(1,276)
Prepaid and other long-term assets	1,702	47
Accounts payable	(1,341)	2,701
Accrued compensation and employee benefits	(538)	(321)
Accrued and other liabilities	10,526	1,521
Deferred revenue	(4,744)	18,214
Other long-term liabilities	1,533	514
Net cash used in operating activities	(95,915)	(33,232)
Investing activities
Purchases of property and equipment	(2,555)	(963)
Proceeds from sale of equipment	11
Purchases of investments	(167,652)	(143,493)
Proceeds from sales of investments		3,000
Proceeds from maturities of investments	162,099	111,016
Net cash used in investing activities	(8,097)	(30,440)
Financing activities
Proceeds from public offering of common stock, net of underwriters discount	108,772
Payment of public offering cost	(358)
Proceeds from issuance of common stock pursuant to equity award plans	6,995	2,967
Net cash provided by financing activities	115,409	2,967
Net increase (decrease) in cash and cash equivalents	11,397	(60,705)
Cash and cash equivalents at beginning of period	57,514	117,773
Cash and cash equivalents at end of period	$ 68,911	$ 57,068