Condensed Combined and Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Operating activities
Net loss	$ (17,479)	$ (5,217)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash research and development expenses	750
Depreciation expense	4	3
Investment premium amortization, net	249
Stock-based compensation expense	4,328	1,017
Interest accrued on notes receivable from stockholder	(2)	(3)
Changes in operating assets and liabilities:
Other assets	(34)	(5)
Prepaid expenses and other current assets	33	(523)
Accounts payable	(37)	512
Income tax payable	(92)	11
Other accrued liabilities	440	542
Accrued compensation	169	186
Net cash used in operating activities	(11,671)	(3,477)
Investing activities
Purchase of short-term investments	(28,618)
Maturities of short-term investments	2,200
Purchase of property and equipment	(10)	(3)
Net cash used in investing activities	(26,428)	(3)
Financing activities
Repayment of notes receivable from stockholder	337
Proceeds from sale of convertible preferred stock	13,500	15,087
Net cash provided by financing activities	12,187	14,963
Increase (decrease) in cash and cash equivalents	(25,912)	11,483
Cash and cash equivalents-beginning of period	51,615	4,207
Cash and cash equivalents-end of period	25,703	15,690
Non-cash financing activities
Issuance of common stock for research and development expenses related to technology licensing option	750
Issuance of Series A-1 convertible preferred stock to Amgen in exchange for license		1,003
Change in obligation to issue Series A-1 convertible preferred stock to Amgen		(1,003)
Issuance of common stock upon vesting of stock awards	65	80
Change in other long-term liabilities related to non-vested stock awards	(65)	251
Restricted stock issued to related party in exchange for notes receivable		331
Offering costs in anticipation of initial public filing included in other accrued liabilities and accounts payable	407
Supplemental cash flow disclosure—Cash paid for taxes	70	13
Convertible Preferred Stock
Financing activities
Offering costs incurred	(19)	(124)
IPO
Financing activities
Offering costs incurred	$ (1,631)