Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Activities
Net loss	$ (116,772)	$ (51,145)	$ (14,562)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities
Depreciation	127	20	31
Amortization of premium and discounts on investment securities	2,024
Deferred income taxes	(214)
Share-based compensation	13,040	2,871	640
Excess tax benefits from share-based compensation	(4,256)
Amortization of debt issuance costs		88	47
Accretion of debt discount		87	59
Non-cash change in fair value of warrant liability		1,181	366
Loss on extinguishment of debt		1,091
Changes in operating assets and liabilities:
Due from Novartis Pharma, AG	(960)
Prepaid expense and other current assets	(2,008)	(6,761)	21
Accrued interest receivable	280
Security deposits	(27)	(96)
Other assets	(100)		1,036
Accrued research and development expenses	5,433	1,472	(485)
Accounts payable and accrued expenses	4,897	2,417	(257)
Deferred revenue	209,624
Net cash provided by (used in) operating activities	111,088	(48,775)	(13,104)
Investing Activities
Purchase of marketable securities	(597,762)
Maturities of marketable securities	171,600
Purchase of property and equipment	(1,655)	(5)
Net cash used in investing activities	(427,817)	(5)
Financing Activities
Payment of debt issuance costs		(43)	(377)
Proceeds from stock option/warrant exercises	2,949	94	2
Proceeds from follow-on public offering, net	55,409
Proceeds from initial public offering, net		175,555
Excess tax benefits from share-based compensation	4,256
Repayment of venture debt facility, net		(11,900)	11,387
Proceeds from issuance of preferred stock, net		49,699
Proceeds from royalty purchase agreement	83,333	41,667
Net cash provided by financing activities	145,947	255,072	11,012
Net change in cash and cash equivalents	(170,782)	206,292	(2,092)
Cash and cash equivalents
Beginning of period	210,596	4,304	6,396
End of period	39,814	210,596	4,304
Supplemental disclosure of cash paid
Income Taxes	40,159
Interest		1,523	437
Supplemental disclosures of non-cash information related to investing activities
Change in unrealized loss in marketable securities, net of tax	$ (65)
Supplemental disclosures of cash flow information
Conversion of preferred stock to common stock upon completion of IPO		174,310
Accreted dividends on Series A, Series A-1, Series of B, B-1 and Series C Preferred Stock		$ 5,891	$ 7,063