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Consolidated Statements of Cash Flows - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Operating activities      
Net loss $ (178,407) $ (128,984) $ (109,577)
Adjustments to reconcile net loss to net cash used in operating activities:      
Depreciation and amortization 735 713 717
Net amortization of premiums and discounts on investments 29 1,187 1,199
Stock-based compensation expense 17,275 20,405 22,283
Amortization of debt discount and issuance costs 1,420    
Change in operating assets and liabilities:      
Prepaid expenses and other current assets (4,663) 342 (120)
Accounts payable (1,380) 909 945
Accrued expenses and other liabilities 11,255 10,070 368
Deferred revenue (8,027) 21,921  
Deferred rent 2,646 (280) (206)
Net cash used in operating activities (159,117) (73,717) (84,391)
Investing activities      
Purchases of property and equipment (2,363) (62) (70)
Proceeds from maturities of investments 137,510 115,544 159,365
Purchases of investments (242,811) (98,374) (134,700)
Net cash (used in) provided by investing activities (107,664) 17,108 24,595
Financing activities      
Proceeds from issuance of convertible senior notes, net of issuance costs 166,885    
Proceeds from issuance of common stock, net of issuance costs 145,705 74,885 50,573
Proceeds from the exercise of stock options and shares issued under employee stock purchase plan 3,519 858 630
Settlements of restricted stock units for tax withholding obligations     (39)
Net cash provided by financing activities 316,109 75,743 51,164
Effect of exchange rate on cash, cash equivalents and restricted cash (78) 211 (66)
Net increase (decrease) in cash, cash equivalents and restricted cash 49,250 19,345 (8,698)
Cash, cash equivalents and restricted cash at beginning of period 69,487 50,142 58,840
Cash, cash equivalents and restricted cash end of period 118,737 69,487 50,142
Reconciliation of cash, cash equivalents and restricted cash reported within the consolidated balance sheets      
Cash and cash equivalents 118,021 68,997 49,663
Short-term restricted cash   200  
Long-term restricted cash 716 290 479
Cash, cash equivalents and restricted cash end of period $ 118,737 $ 69,487 $ 50,142