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Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Investment in Commercial Real Estate Properties |  |  |  |  |
Rental revenue increase (decrease) from amortization of acquired leases | $ (300,000) | $ 1,600,000 | $ (4,000,000) | $ 4,500,000 |
Identified intangible assets (included in other assets): |  |  |  |  |
Gross amount | 226,709,000 |  | 226,709,000 |  | $ 215,673,000
Accumulated amortization | (191,758,000) |  | (191,758,000) |  | (190,523,000)
Net | 34,951,000 |  | 34,951,000 |  | 25,150,000
Identified intangible liabilities (included in deferred revenue): |  |  |  |  |
Gross amount | 225,029,000 |  | 225,029,000 |  | 241,409,000
Accumulated amortization | (218,371,000) |  | (218,371,000) |  | (230,479,000)
Net | 6,658,000 |  | 6,658,000 |  | 10,930,000
Net intangible assets reclassified to assets held for sale | 0 |  | 0 |  | 0
Net intangible liabilities reclassified to liabilities relates to assets held for sale | $ 0 |  | $ 0 |  | $ 0
Minimum | Above-market leases |  |  |  |  |
Investment in Commercial Real Estate Properties |  |  |  |  |
Estimated useful life of other intangible assets (in years) |  |  | 1 year |  |
Minimum | Below-market leases |  |  |  |  |
Investment in Commercial Real Estate Properties |  |  |  |  |
Estimated useful life of other intangible assets (in years) |  |  | 1 year |  |
Minimum | In-place leases |  |  |  |  |
Investment in Commercial Real Estate Properties |  |  |  |  |
Estimated useful life of other intangible assets (in years) |  |  | 1 year |  |
Maximum | Above-market leases |  |  |  |  |
Investment in Commercial Real Estate Properties |  |  |  |  |
Estimated useful life of other intangible assets (in years) |  |  | 15 years |  |
Maximum | Below-market leases |  |  |  |  |
Investment in Commercial Real Estate Properties |  |  |  |  |
Estimated useful life of other intangible assets (in years) |  |  | 15 years |  |
Maximum | In-place leases |  |  |  |  |
Investment in Commercial Real Estate Properties |  |  |  |  |
Estimated useful life of other intangible assets (in years) |  |  | 15 years |  |
Building | Minimum |  |  |  |  |
Investment in Commercial Real Estate Properties |  |  |  |  |
Estimated useful life (in years) |  |  | 3 years |  |
Building | Maximum |  |  |  |  |
Investment in Commercial Real Estate Properties |  |  |  |  |
Estimated useful life (in years) |  |  | 40 years |  |

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