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SEC Schedule, Article 12-28, Real Estate and Accumulated Depreciation Roll Forward (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate [Roll Forward] |  |  |
Balance at the beginning of the year | $ 22,844,697 | $ 21,605,545 | $ 21,058,714
Additions to / improvements of real estate | 996,989 | 1,671,898 | 1,043,379
Assets sold/written-off | (538,383) | (432,746) | (496,548)
Balance at the end of the year | 23,303,303 | 22,844,697 | 21,605,545
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation [Roll Forward] |  |  |
Balance at the beginning of the year | 5,239,179 | 4,871,102 | 4,566,570
Depreciation expense | 573,898 | 564,938 | 533,342
Assets sold/written-off | (311,440) | (196,861) | (228,810)
Balance at the end of the year | 5,501,637 | 5,239,179 | 4,871,102
Boston Properties Limited Partnership |  |  |
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate [Roll Forward] |  |  |
Balance at the beginning of the year | 22,449,476 | 21,207,189 | 20,647,236
Additions to / improvements of real estate | 996,989 | 1,671,898 | 1,043,379
Assets sold/written-off | (519,971) | (429,611) | (483,426)
Balance at the end of the year | 22,926,494 | 22,449,476 | 21,207,189
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation [Roll Forward] |  |  |
Balance at the beginning of the year | 5,135,289 | 4,773,800 | 4,473,895
Depreciation expense | 566,813 | 557,130 | 525,584
Assets sold/written-off | (305,991) | (195,641) | (225,679)
Balance at the end of the year | $ 5,396,111 | $ 5,135,289 | $ 4,773,800

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- Definition Real Estate Accumulated Depreciation Excluding Furniture Fixtures and Equipment + References No definition available. + Details
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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