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Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019
Investment in Commercial Real Estate Properties |  |  |  |  |
Rental revenue from amortization of acquired leases | $ 1,600,000 | $ 1,200,000 | $ 4,500,000 | $ 3,600,000 |
Identified intangible assets (included in other assets): |  |  |  |  |
Gross amount | 224,221,000 |  | 224,221,000 |  | $ 255,198,000
Accumulated amortization | (208,135,000) |  | (208,135,000) |  | (228,223,000)
Net | 16,086,000 |  | 16,086,000 |  | 26,975,000
Identified intangible liabilities (included in deferred revenue): |  |  |  |  |
Gross amount | 273,240,000 |  | 273,240,000 |  | 282,048,000
Accumulated amortization | (255,879,000) |  | (255,879,000) |  | (249,514,000)
Net | $ 17,361,000 |  | $ 17,361,000 |  | 32,534,000
Net intangible assets reclassified to assets held for sale |  |  |  |  | 0
Net intangible liabilities reclassified to liabilities relates to assets held for sale |  |  |  |  | $ 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) |  |  | 14 years |  |
Maximum | Below-market leases |  |  |  |  |
Investment in Commercial Real Estate Properties |  |  |  |  |
Estimated useful life of other intangible assets (in years) |  |  | 14 years |  |
Maximum | In-place leases |  |  |  |  |
Investment in Commercial Real Estate Properties |  |  |  |  |
Estimated useful life of other intangible assets (in years) |  |  | 14 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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