CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Cash flows from operating activities:    
Net loss $ 48,486 $ 32,581
Adjustments to reconcile net loss to net cash used in operating activities:    
Stock-based compensation expense 8,488 9,478
Depreciation and amortization expense 1,789 661
Accretion of discount on investments (29) (489)
Non-cash interest expense 87 21
Changes in operating assets and liabilities:    
Prepaid expenses and other current assets 914 2,695
Operating lease, right-of-use-asset 1,579 509
Accounts payable 905 (966)
Accrued expenses and other current liabilities (3,954) (2,487)
Operating lease liabilities (1,308) 287
Net cash used in operating activities (40,015) (22,872)
Cash flows from investing activities:    
Purchases of property, plant and equipment (2,851) (4,164)
Purchases of investments (10,001) (117,983)
Sales and maturities of investments 105,000 29,155
Net cash provided by (used in) investing activities 92,148 (92,992)
Cash flows from financing activities:    
Proceeds from issuance of common stock upon exercise of stock options 486 799
Net cash provided by financing activities 486 799
Net increase (decrease) in cash, cash equivalents and restricted cash 52,619 (115,065)
Cash, cash equivalents and restricted cash at beginning of period 93,633 309,421
Cash, cash equivalents and restricted cash at end of period 146,252 194,356
Supplemental cash flow information:    
Cash paid for interest, net of interest capitalized 895 336
Cash paid for leases 2,147 177
Supplemental disclosure of non-cash investing and financing information:    
Lease assets obtained in exchange for new operating lease liabilities   27,944
Lease asset derecognized upon lease cancellation 982  
Purchases of property, plant and equipment included in accounts payable or accrued expenses $ 923 $ 4,630