Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Activities
Net loss	$ (105,717)	$ (116,772)	$ (51,145)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities
Depreciation	698	127	20
Amortization of debt issuance costs			88
Accretion of debt discount			87
Amortization of premium and discounts on investment securities	2,846	2,024
Gain on sale of marketable securities	(57)
Non-cash change in fair value of warrant liability			1,181
Loss on extinguishment of debt			1,091
Deferred income taxes	(17,341)	(214)
Share-based compensation	24,760	13,040	2,871
Excess tax benefits from share-based compensation	(1,302)	(4,256)
Changes in operating assets and liabilities:
Due from Novartis Pharma AG	(3,429)	(960)
Prepaid expense and other current assets	3,308	(2,008)	(6,761)
Accrued interest receivable	155	280
Other assets	(107)	(127)	(96)
Accrued research and development expenses	10,902	5,433	1,472
Accounts payable and accrued expenses	3,311	4,897	2,417
Deferred revenue	3,442	209,624
Net cash (used in) provided by operating activities	(78,531)	111,088	(48,775)
Investing Activities
Purchase of marketable securities	(411,565)	(597,762)
Sale of marketable securities	395,977
Maturities of marketable securities	266,000	171,600
Purchase of property and equipment	(2,615)	(1,655)	(5)
Proceeds from sale of assets	6
Net cash provided by (used in) investing activities	247,803	(427,817)	(5)
Financing Activities
Payment of debt issuance costs			(43)
Proceeds from stock option/warrant exercises	11,473	2,949	94
Proceeds from follow-on public offering, net		55,409
Proceeds from initial public offering, net			175,555
Excess tax benefits from share-based compensation	1,302	4,256
Repayment of venture debt facility, net			(11,900)
Proceeds from issuance of preferred stock, net			49,699
Proceeds from royalty purchase agreement		83,333	41,667
Net cash provided by financing activities	12,775	145,947	255,072
Net change in cash and cash equivalents	182,047	(170,782)	206,292
Cash and cash equivalents
Beginning of period	39,814	210,596	4,304
End of period	221,861	39,814	210,596
Supplemental disclosure of cash paid
Interest			1,523
Income taxes, net of refunds received	399	40,159
Supplemental disclosure of non-cash information related to investing activities
Change in unrealized loss on available for sale securities, net of tax	$ (408)	$ (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