Consolidated and Combined Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (57,221)	$ (28,006)	$ (8,773)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	10,251	10,101	1,713
Amortization of investment premiums and discounts	3,465	526
Non-cash research and development expenses		750
Depreciation expense	48	6	4
Write off of property and equipment	21
Loss on foreign exchange	94
Interest accrued on notes receivable from stockholder		(2)	(4)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(767)	(1,246)	(158)
Other assets	(61)	(37)	27
Accounts payable	1,005	(164)	485
Accrued compensation	1,399	894	280
Accrued research and development expenses	4,288	468	356
Other accrued liabilities	293	4	104
Long-term liabilities	29	78
Net cash used in operating activities	(37,156)	(16,628)	(5,966)
Investing activities
Purchase of short-term investments	(379,776)	(95,525)
Sales and maturities of short-term investments	160,133	12,208
Transfer to restricted cash	(194)
Purchase of property and equipment	(290)	(46)	(3)
Net cash used in investing activities	(220,127)	(83,363)	(3)
Financing activities
Proceeds from sale of common stock, net of offering costs	263,434	56,455
Taxes paid related to net share settlement of restricted stock units	(4,647)
Proceeds from exercise of stock options	439
Proceeds from sale of convertible preferred stock, net of offering costs		13,481	53,377
Repayment of notes receivable from stockholder		337
Net cash provided by financing activities	259,226	70,273	53,377
Effect of exchange rates on cash	(94)
Increase (decrease) in cash and cash equivalents	1,849	(29,718)	47,408
Cash and cash equivalents-beginning of period	21,897	51,615	4,207
Cash and cash equivalents-end of period	23,746	21,897	51,615
Non-cash financing activities
Issuance of common stock related to technology licensing option	750	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	80	90	105
Change in other long-term liabilities related to non-vested stock awards	(80)	(90)	226
Restricted stock issued to related party in exchange for notes receivable			331
Supplemental cash flow disclosure
Cash paid for taxes	$ 3	$ 70	$ 22