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Significant Accounting Policies (Tables) | 9 Months Ended
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Sep. 30, 2021
Accounting Policies [Abstract] |
Summary of identified intangible assets (acquired above-market leases and in-place leases) and intangible liabilities (acquired below-market leases) | The following summarizes our identified intangible assets (acquired above-market leases and in-place leases) and intangible liabilities (acquired below-market leases) as of September 30, 2021 and December 31, 2020 (in thousands):
| September 30, 2021 |  | December 31, 2020
Identified intangible assets (included in other assets): |  |  |
Gross amount | $ | 226,709 |  |  | $ | 215,673 |
Accumulated amortization | (191,758) |  |  | (190,523) |
Net (1) | $ | 34,951 |  |  | $ | 25,150 |
Identified intangible liabilities (included in deferred revenue): |  |  |
Gross amount | $ | 225,029 |  |  | $ | 241,409 |
Accumulated amortization | (218,371) |  |  | (230,479) |
Net (1) | $ | 6,658 |  |  | $ | 10,930 |
(1) As of September 30, 2021 and December 31, 2020, no net intangible assets and no net intangible liabilities were reclassified to assets held for sale or liabilities related to assets held for sale.
Schedule of marketable securities | As of September 30, 2021 and December 31, 2020, we held the following marketable securities (in thousands):
| September 30, 2021 |  | December 31, 2020
Commercial mortgage-backed securities | $ | 24,355 |  |  | $ | 28,570 |
Total marketable securities available-for-sale | $ | 24,355 |  |  | $ | 28,570 |
Equity marketable securities | $ | 10,073 |  |  | $ | — |
Total investment in marketable securities | $ | 34,428 |  |  | $ | 28,570 |
Schedules of concentration of risk, by risk factor | For the three months ended September 30, 2021, the following properties contributed more than 5.0% of our annualized cash rent from office properties, including our share of annualized cash rent from joint venture office properties:
Property | Three months ended September 30, 2021
11 Madison Avenue | 10.5%
420 Lexington Ave (Graybar) | 8.0%
1515 Broadway | 7.9%
1185 Avenue of the Americas | 7.6%
280 Park Avenue | 6.5%
919 Third Avenue | 5.2%
485 Lexington Avenue | 5.1%
555 West 57th Street | 5.1%
100 Church Street | 5.0%

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- Definition Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 + Details
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