Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating activities
Net loss	$ (36,854)	$ (30,544)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,150	659
Stock-based compensation expense	13,237	3,886
Net amortization of premium on investments	319	305
Changes in operating assets and liabilities:
Collaboration receivable - related party	(3,982)	476
Other receivables	(2,948)
Prepaid expenses and other assets	125	(791)
Accounts payable	(1,210)	1,684
Accrued expenses and other liabilities	(2,558)	(1,130)
Deferred rent	13,689	(55)
Refundable income taxes and income taxes payable	3,841	(5,303)
Deferred revenue - related party	(6,414)	2,702
Net cash used in operating activities	(21,605)	(28,111)
Investing activities
Purchases of marketable securities	(106,205)	(126,338)
Proceeds from maturities and sales of marketable securities	198,078	95,501
Purchases of property and equipment	(14,679)	(99)
Net cash provided by (used in) investing activities	77,194	(30,936)
Financing activities
Net proceeds from public offering of common stock, (payment of) commissions and offering costs	(207)	94,698
Net proceeds from stock option exercises and employee stock purchase plan	3,120	1,036
Net cash provided by financing activities	2,913	95,734
Net increase in cash and cash equivalents	58,502	36,687
Cash and cash equivalents at beginning of the period	14,031	71,560
Cash and cash equivalents at end of the period	72,533	108,247
Supplemental cash flow information
Cash paid for income taxes		5,958
Supplemental disclosure of non-cash investing and financing transactions
Additions to property and equipment in accounts payable and accrued expenses	1,531	12
Vesting of restricted stock	(4)	(6)
Public offering costs in accounts payable and accrued expenses		$ 13
Proceeds from stock option exercises in other current assets	$ 3