Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating activities
Net income (loss)	$ (83,352)	$ 11,366	$ 19,984
Adjustments to reconcile net income (loss) to cash used in operating activities:
Depreciation and amortization	1,359	1,389	1,382
Noncash interest expense			7
Amortization of premium on investment securities	2,333	1,469	751
Stock-based compensation expense	4,974	2,809	2,353
Revaluation of convertible preferred stock warrant liability	(24)	(83)	(23)
Unrealized gain on foreign currency forward contracts	(261)	(51)
Changes in operating assets and liabilities:
Receivables from collaborations	353	293	(955)
Prepaid expenses and other current assets	(469)	(2,481)	1,615
Other assets	396	(2,091)
Accounts payable	(1,773)	2,017	1,437
Accrued compensation and employee benefits	650	(980)	117
Accrued and other liabilities	11,908	3,375	(171)
Deferred revenue	1,169	(65,426)	(35,387)
Other long-term liabilities	(878)	(831)	45
Accrued income taxes			(2,476)
Net cash used in operating activities	(63,615)	(49,225)	(11,321)
Investing activities
Decrease in restricted cash			6,000
Purchases of property and equipment	(933)	(362)	(1,477)
Purchases of investments	(219,813)	(144,644)	(33,805)
Proceeds from sales of investments	8,009	36,517	2,163
Proceeds from maturities of investments	92,001	40,687	59,829
Net cash (used in) provided by investing activities	(120,736)	(67,802)	32,710
Financing activities
Proceeds from public offering of common stock, net of underwriters discount	251,865
Payment of public offering costs	(5,883)
Proceeds from issuance of common stock, including early exercise of stock options	2,529	317	167
Repurchase of unvested common stock			(51)
Proceeds from issuance of convertible preferred stock, net of issuance costs			96,700
Repayment of debt			(2,598)
Net cash provided by financing activities	248,511	317	94,218
Net increase (decrease) in cash and cash equivalents	64,160	(116,710)	115,607
Cash and cash equivalents at beginning of year	53,613	170,323	54,716
Cash and cash equivalents at end of year	117,773	53,613	170,323
Supplemental disclosure of cash flow information
Interest paid			14
Income taxes paid		57	2,476
Noncash investing activities
Net change in accounts payable related to purchase of property and equipment	$ 165		$ (362)