Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating Activities
Net loss	$ (30,495)	$ (93,672)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities
Depreciation	459	43
Amortization of premium and discounts on investment securities	2,068	802
Deferred income taxes	(4,098)	1,226
Share-based compensation	11,433	5,912
Excess tax benefits from share-based compensation		(1,244)
Changes in operating assets and liabilities:
Due from Novartis Pharma, AG	713
Prepaid expense and other current assets	(2,073)	2,717
Accrued interest receivable	431	78
Security deposits	(185)
Other assets	48
Accrued research and development expenses	1,857	(702)
Accounts payable and accrued expenses	(2,557)	229
Income tax payable		29,559
Deferred revenue	6,724	200,000
Net cash (used in) provided by operating activities	(15,675)	144,948
Investing Activities
Purchase of marketable securities	(221,918)	(244,824)
Maturities of marketable securities	231,000	40,000
Purchase of property and equipment	(719)	(842)
Proceeds from sale of assets	6
Net cash provided by (used in) investing activities	8,369	(205,666)
Financing Activities
Proceeds from stock option/warrant exercises	4,212	300
Proceeds from follow-on public offering, net		55,409
Excess tax benefits from share-based compensation		1,244
Proceeds from royalty purchase agreement		41,667
Net cash provided by financing activities	4,212	98,620
Net change in cash and cash equivalents	(3,094)	37,902
Cash and cash equivalents
Beginning of period	39,814	210,596
End of period	36,720	248,498
Supplemental disclosures of non-cash information related to investing activities
Change in unrealized gains in available for sale securities, net of tax	$ 89	$ 25