Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	$ (48,506)	$ (49,800)
Adjustments to reconcile net loss to net cash used in operating activities:
Acquired in-process research and development		17,000
Depreciation expense	99	63
Stock-based compensation expense	3,920	2,538
Non-cash interest expense	2,200
Loss on disposal of property and equipment		80
Changes in operating assets and liabilities:
Other assets	(352)	200
Accounts payable	(464)	486
Accrued expenses	1,277	2,310
Other liabilities	1,526	(16)
Net cash used in operating activities	(40,300)	(27,139)
Investing activities
Acquisition of product candidate and technology licenses and milestone payments		(17,000)
Purchase of property and equipment	(172)	(604)
Net cash used in investing activities	(172)	(17,604)
Financing activities
Proceeds from sale of common stock, net of issuance costs	179,950	94,199
Proceeds from exercise of stock options	2,807	519
Net cash provided by financing activities	182,757	94,718
Increase in cash and cash equivalents	142,285	49,975
Cash and cash equivalents at beginning of period	256,861	130,310
Cash and cash equivalents at end of period	399,146	180,285
Non-cash investing and financing activities
Common stock issuance costs - cash not paid as of period end	204
Purchase of property and equipment - cash not paid as of period end		$ 340