Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Operating activities
Net loss	$ (97,869)	$ (58,290)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	1,142	1,015
Amortization of premium on investment securities	2,669	1,523
Stock-based compensation expense	6,876	3,457
Revaluation of convertible preferred stock warrant liability		(24)
Unrealized (gain) loss on foreign currency forward contracts	114	(97)
Changes in operating assets and liabilities:
Receivables from collaborators	236	435
Prepaid expenses and other current assets	(797)	(540)
Prepaid and other long-term assets	(9,954)	361
Accounts payable	9,354	(3,323)
Accrued compensation and employee benefits	(40)	3
Accrued and other liabilities	(2,583)	10,004
Deferred revenue	33,787	3,225
Other long-term liabilities	418	(655)
Net cash used in operating activities	(56,647)	(42,906)
Investing activities
Purchases of property and equipment	(1,329)	(582)
Purchases of investments	(156,009)	(152,793)
Proceeds from sales of investments		6,644
Proceeds from maturities of investments	145,661	68,211
Net cash used in investing activities	(11,677)	(78,520)
Financing activities
Proceeds from initial public offering, net of underwriters discount		131,026
Payment of public offering costs	(239)	(5,025)
Proceeds from issuance of common stock	3,934	522
Net cash provided by financing activities	3,695	126,523
Net (decrease) increase in cash and cash equivalents	(64,629)	5,097
Cash and cash equivalents at beginning of period	117,773	53,613
Cash and cash equivalents at end of period	$ 53,144	$ 58,710