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Significant Accounting Policies - Acquired Real Estate Leases and Amortization (Details) | 12 Months Ended
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Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) item | Dec. 31, 2015 USD ($) item
Acquired real estate leases and amortization |  |  |
Number of acquisitions of properties with leases | item |  | 3 | 1
Acquired in-place and above market real estate leases |  |  |
Acquired real estate leases and amortization |  |  |
Amortization expense | $ 38,970,000 | $ 36,854,000 | $ 38,829,000
Estimated annual amortization for succeeding five years |  |  |
2018 | 27,214,000 |  |
2019 | 18,644,000 |  |
2020 | 12,721,000 |  |
2021 | 9,022,000 |  |
2022 | 5,427,000 |  |
2023 and thereafter | $ 13,492,000 |  |
Acquired in-place and above market real estate leases | Minimum |  |  |
Acquired real estate leases and amortization |  |  |
Term of lease | 6 months |  |
Acquired in-place and above market real estate leases | Maximum |  |  |
Acquired real estate leases and amortization |  |  |
Term of lease | 281 months |  |

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