Consolidated and Combined Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (79,049)	$ (57,221)	$ (28,006)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	16,784	10,251	10,101
Amortization of investment premiums and discounts	2,582	3,465	526
Depreciation expense	383	48	6
Loss on foreign exchange	0	94	0
Write-off of property and equipment	0	21	0
Interest accrued on notes receivable from stockholder	0	0	(2)
Non-cash research and development expenses	0	0	750
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(742)	(767)	(1,246)
Other assets	6	(61)	(37)
Accounts payable	981	1,005	(164)
Accrued compensation	1,121	1,399	894
Accrued research and development expenses	(2,704)	4,288	468
Other accrued liabilities	215	293	4
Long-term liabilities	398	29	78
Net cash used in operating activities	(60,025)	(37,156)	(16,628)
Investing activities
Purchases of short-term investments	(304,928)	(379,776)	(95,525)
Sales of short-term investments	242,643	64,020	5,808
Maturities of short-term investments	149,046	96,113	6,400
Transfer to restricted cash	0	(194)	0
Purchases of property and equipment	(3,020)	(290)	(46)
Net cash provided by (used in) investing activities	83,741	(220,127)	(83,363)
Financing activities
Proceeds from sale of common stock, net of offering costs	0	263,434	56,455
Taxes paid related to net share settlement of restricted stock units	(94)	(4,647)	0
Proceeds from employee stock awards	600	439	0
Proceeds from sale of convertible preferred stock, net of offering costs	0	0	13,481
Repayment of notes receivable from stockholder	0	0	337
Net cash provided by financing activities	506	259,226	70,273
Effect of exchange rates on cash	0	(94)	0
Increase in cash and cash equivalents	24,222	1,849	(29,718)
Cash and cash equivalents at beginning of period	23,746	21,897	51,615
Cash and cash equivalents at end of period	47,968	23,746	21,897
Non-cash investing and financing activities
Issuance of common stock related to technology licensing option	0	0	750
Issuance of common stock upon vesting of stock awards	60	80	90
Change in long-term liabilities related to non-vested stock awards	(60)	(80)	(90)
Property and equipment purchases included in liabilities	352	0	0
Supplemental cash flow disclosure
Cash paid for taxes	$ 10	$ 3	$ 70