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CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands | 6 Months Ended
--- | ---
Jun. 30, 2019 | Jun. 30, 2018
Cash flows from operating activities |  |
Net loss | $ (76,784) | $ (92,780)
Adjustments to reconcile net loss to net cash used in operating activities: |  |
Depreciation and amortization expense | 5,483 | 2,540
Stock-based compensation | 8,683 | 7,595
Changes in operating assets and liabilities: |  |
Accounts receivable | 23,971 | (4,194)
Prepaid expenses | (3,209) | (22)
Other assets | (21,091) | (5,511)
Accounts payable | (2,598) | (684)
Accrued expenses | (2,663) | 8,215
Lease exit liability | 0 | (298)
Deferred revenue | (10,861) | (3,371)
Lease liabilities | 231 | 0
Deferred rent | 0 | (516)
Net cash used in operating activities | (78,838) | (89,026)
Cash flows from investing activities |  |
Purchases of marketable securities | (142,305) | (86,491)
Proceeds from sale and maturities of marketable securities | 63,955 | 118,090
Purchases of property and equipment | (2,262) | (20,023)
Net cash provided by (used in) investing activities | (80,612) | 11,576
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 | 981 | 1,125
Net cash provided by financing activities | 119,638 | 104,384
Net change in cash and cash equivalents | (39,812) | 26,934
Cash and cash equivalents at beginning of period | 220,128 | 211,727
Cash and cash equivalents at end of period | 180,316 | 238,661
Supplemental Cash Flow Information [Abstract] |  |
Right-of-use assets modified in exchange for operating lease obligations | $ 1,988 | $ 0

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