Consolidated 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 loss	$ (39,407)	$ (20,102)	$ (23,706)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,440	1,179	801
Net loss on disposal of fixed assets		10
Stock-based compensation expense	3,030	742	371
Deferred taxes	3,842	6,707	(10,660)
Net amortization of premiums and discounts on investments	284	287	391
Changes in operating assets and liabilities:
Unbilled collaboration receivable	(476)
Prepaid expenses and other assets	(1,562)	(94)	(63)
Accounts payable	30	(322)	809
Accrued expenses and other liabilities	4,878	156	525
Deferred rent	(85)	(46)	243
Income taxes payable	(3,302)	(12,993)	17,867
Deferred revenue	(25,072)	(25,072)	(1,797)
Net cash used in operating activities	(56,400)	(49,548)	(15,219)
Investing activities
Purchases of marketable securities	(146,049)	(88,524)	(105,936)
Proceeds from maturities and sales of marketable securities	60,126	113,041	85,482
Purchases of property and equipment	(1,294)	(1,475)	(1,906)
Net cash (used in) provided by investing activities	(87,217)	23,042	(22,360)
Financing activities
Proceeds from initial public offering, net of commissions	113,367
Proceeds from private placement	12,750
Payment of initial public offering costs	(2,387)
Net proceeds from issuance of Series C convertible preferred stock			77,301
Net proceeds from stock option exercises and issuance of common and restricted common stock	150	142	57
Net cash provided by financing activities	123,880	142	77,358
Net (decrease) increase in cash and cash equivalents	(19,737)	(26,364)	39,779
Cash and cash equivalents at beginning of the period	91,297	117,661	77,882
Cash and cash equivalents at end of the period	71,560	91,297	117,661
Supplemental cash flow information
Cash paid for income taxes		3,549
Supplemental disclosure of non-cash investing and financing transactions:
Conversion of convertible preferred stock upon initial public offering	115,923
Vesting of restricted stock	64
Additions to property, plant and equipment included in accounts payable	$ 339	$ 57	$ 275