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Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Real Estate [Line Items] |  |  |
Weighted average amortization period, above-market lease costs | 1 year 8 months 12 days |  |
Weighted average amortization period, below-market lease costs | 2 years 9 months 18 days |  |
Weighted average amortization period, in-place lease costs | 3 years 6 months |  |
Period from cessation of major construction activity to completion of construction project, maximum | 1 year |  |
Depreciation expense (including amortization of the capital lease asset) | $ 233,500,000 | $ 242,800,000 | $ 365,300,000
Amortization of above and below Market Leases | 4,500,000 | 6,800,000 | 20,300,000
Reduction in interest expense from amortization of above-market rate mortgages | 0 | 0 | $ 800,000
Identified intangible assets (included in other assets): |  |  |
Gross amount | 255,198,000 | 266,540,000 |
Accumulated amortization | (228,223,000) | (241,040,000) |
Net | 26,975,000 | 25,500,000 |
Identified intangible liabilities (included in deferred revenue): |  |  |
Gross amount | 282,048,000 | 276,245,000 |
Accumulated amortization | (249,514,000) | (253,767,000) |
Net | 32,534,000 | 22,478,000 |
Net intangible assets reclassed to assets held for sale | 0 | 0 |
Net intangible liabilities reclassed to liabilities relates to assets held for sale | 0 | $ 0 |
Other Identifiable Assets |  |  |
Estimated annual amortization |  |  |
2020 | 5,183,000 |  |
2021 | 3,638,000 |  |
2022 | 2,064,000 |  |
2023 | 1,435,000 |  |
2024 | 1,040,000 |  |
Acquired above-market leases, net of acquired below-market leases |  |  |
Below Market Lease, Net, Amortization Income, Fiscal Year Maturity [Abstract] |  |  |
2020 | (4,320,000) |  |
2021 | (2,748,000) |  |
2022 | (2,224,000) |  |
2023 | (1,301,000) |  |
2024 | $ (834,000) |  |
Minimum | Above Market Leases |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life of other intangible assets (in years) | 1 year |  |
Minimum | Below Market Leases |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life of other intangible assets (in years) | 1 year |  |
Minimum | In Place Leases |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life of other intangible assets (in years) | 1 year |  |
Maximum | Above Market Leases |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life of other intangible assets (in years) | 14 years |  |
Maximum | Below Market Leases |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life of other intangible assets (in years) | 14 years |  |
Maximum | In Place Leases |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life of other intangible assets (in years) | 14 years |  |
Building (fee ownership) |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life (in years) | 40 years |  |
Building (leasehold interest) | Maximum |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life (in years) | 40 years |  |
Right of use assets - financing leases | Maximum |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life (in years) | 40 years |  |
Furniture and fixtures | Minimum |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life (in years) | 4 years |  |
Furniture and fixtures | Maximum |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life (in years) | 7 years |  |
Building | Minimum |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life (in years) | 3 years |  |
Building | Maximum |  |  |
Real Estate [Line Items] |  |  |
Estimated useful life (in years) | 40 years |  |

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