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Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Investment in Commercial Real Estate Properties |  |  |  |  |
Depreciable real estate reserves | $ 0 | $ (29,064) | $ 0 | $ (85,336) |
Rental revenue from amortization of acquired leases | 1,800 | 6,400 | 4,200 | 11,200 |
Reduction in interest expense from amortization of above-market rate mortgages | 0 | $ 700 | 0 | $ 1,300 |
Identified intangible assets (included in other assets): |  |  |  |  |
Gross amount | 263,138 |  | 263,138 |  | $ 325,880
Accumulated amortization | (239,344) |  | (239,344) |  | (277,038)
Net | 23,794 |  | 23,794 |  | 48,842
Identified intangible liabilities (included in deferred revenue): |  |  |  |  |
Gross amount | 274,120 |  | 274,120 |  | 540,283
Accumulated amortization | (249,905) |  | (249,905) |  | (402,583)
Net | 24,215 |  | 24,215 |  | 137,700
Net intangible assets reclassed to assets held for sale | 100 |  | 100 |  | 13,900
Net intangible liabilities reclassed to liabilities relates to assets held for sale | $ 100 |  | $ 100 |  | $ 4,100
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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