CONDENSED STATEMENTS OF CASH FLOWS - USD ($)
$ in Thousands
 11 Months Ended 12 Months Ended
Dec. 31, 2014
Dec. 31, 2016
Dec. 31, 2015
Cash flows from operating activities      
Net loss $ (161) $ (36,375) $ (31,335)
Adjustments to reconcile net loss to net cash used in operating activities:      
Depreciation   594 148
Amortization/accretion related to marketable securities   648 (41)
Stock-based compensation   3,828 428
Change in fair value of convertible preferred stock liability     17,600
Other     40
Changes in operating assets and liabilities:      
Prepaid and other current assets (12) 290 (1,265)
Other assets   (869)  
Accounts payable 57 518 1,240
Accrued and other liabilities 17 2,814 1,218
Other long-term liabilities 3 695 639
Net cash used in operating activities (96) (27,857) (11,328)
Cash flows from investing activities      
Purchases of marketable securities   (258,281) (104,385)
Sales of marketable securities   4,199  
Maturities of marketable securities   213,725 14,100
Purchase of property and equipment   (2,199) (1,747)
Net cash used in investing activities   (42,556) (92,032)
Cash flows from financing activities      
Proceeds from issuance of common stock in IPO, net of issuance costs 2 71,354  
Proceeds from issuance of convertible preferred stock, net of issuance costs 12,611   95,569
Payment of offering costs     (729)
Proceeds from exercise of common stock options   4 108
Net cash provided by financing activities 12,613 71,358 94,948
Net increase in cash and cash equivalents 12,517 945 (8,412)
Cash and cash equivalents at beginning of the period   4,105 12,517
Cash and cash equivalents at end of the period 12,517 5,050 4,105
Supplemental disclosures of cash flow information      
Convertible preferred stock issuance costs incurred but not paid 36    
Purchases of property and equipment incurred but not paid   $ 84 286
Convertible preferred stock liability $ 2,600    
Deferred offering costs incurred but not paid     $ 222