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CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands | 9 Months Ended
--- | ---
Sep. 30, 2019 | Sep. 30, 2018
Cash flows from operating activities |  |
Net loss | $ (121,415) | $ (126,809)
Adjustments to reconcile net loss to net cash used in operating activities: |  |
Depreciation and amortization expense | 8,119 | 5,268
Stock-based compensation | 14,081 | 12,168
Changes in operating assets and liabilities: |  |
Accounts receivable | 20,080 | (7,328)
Prepaid expenses | (5,612) | (1,164)
Other assets | (2,621) | (9,293)
Accounts payable | (913) | (1,465)
Accrued expenses | (4,678) | 9,460
Lease exit liability | 0 | (298)
Deferred revenue | (7,114) | (5,807)
Lease liabilities | 3,847 | 0
Deferred rent | 0 | (738)
Net cash used in operating activities | (96,226) | (126,006)
Cash flows from investing activities |  |
Purchases of marketable securities | (214,178) | (120,039)
Proceeds from sale and maturities of marketable securities | 130,236 | 155,848
Purchases of property and equipment | (3,042) | (24,239)
Net cash provided by (used in) investing activities | (86,984) | 11,570
Cash flows from financing activities |  |
Proceeds from issuance of common stock, net of offering costs | 118,657 | 103,259
Proceeds from stock option exercises and ESPP purchases | 1,018 | 1,257
Net cash provided by financing activities | 119,675 | 104,516
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect, Total | (63,535) | (9,920)
Cash and cash equivalents at beginning of period | 220,128 | 211,727
Cash and cash equivalents at end of period | 156,593 | 201,807
Supplemental Cash Flow Information [Abstract] |  |
Right-of-use assets modified in exchange for operating lease obligations | $ 6,408 | $ 0

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