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Real Estate Narrative (Details) $ in Thousands |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Feb. 14, 2019 ft² yr | Jan. 24, 2019 USD ($) ft² a | Jan. 23, 2019 USD ($) | Mar. 31, 2019 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) | Apr. 18, 2019 USD ($) ft² | Jan. 10, 2019 USD ($) ft²
Real Estate [Line Items] |  |  |  |  |  |  |  |
Right of use assets - finance leases |  |  |  | $ 187,292 |  | $ 0 |  |
Proceeds from sales of real estate |  |  |  | 20,019 | $ 116,120 |  |  |
Gains (loss) on sales of real estate |  |  |  | 905 | (96,397) |  |  |
Boston Properties Limited Partnership |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Right of use assets - finance leases |  |  |  | 187,292 |  | 0 |  |
Proceeds from sales of real estate |  |  |  | 20,019 | 116,120 |  |  |
Gains (loss) on sales of real estate |  |  |  | 905 | (98,907) |  |  |
325 Main Street [Member] |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 115,000 |  |  |  |  |  |  |
Platform 16 [Member] |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  | 1,100,000 |  |  |  |  |  |
Finance Lease fixed purchase price |  | $ 134,800 |  |  |  |  |  |
Right of use assets - finance leases |  | $ 122,600 |  |  |  |  |  |
Lessee, Finance Lease, Term of Contract |  | 65 years |  |  |  |  |  |
Area of Land | a |  | 5.6 |  |  |  |  |  |
Carnegie Center [Member] |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  | 1,700,000
Aggregate purchase price |  |  |  |  |  |  |  | $ 51,500
Deferred purchase price |  |  |  |  |  |  |  | $ 8,600
One Tower Center [Member] |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Impairment loss |  |  |  | 24,000 |  |  |  |
One Tower Center [Member] | Boston Properties Limited Partnership |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Impairment loss |  |  |  | 22,300 |  |  |  |
2600 Tower Oaks Boulevard [Member] |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  | 179,000 |  |  |  |  |  |
Gross Sale Price |  | $ 22,700 |  |  |  |  |  |
Proceeds from sales of real estate |  | $ 21,400 |  |  |  |  |  |
Net income (loss) |  |  | $ (200) |  | $ (200) |  |  |
Gains (loss) on sales of real estate |  |  |  | $ 600 |  |  |  |
Impairment loss |  |  |  |  |  | 3,100 |  |
2600 Tower Oaks Boulevard [Member] | Boston Properties Limited Partnership |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Impairment loss |  |  |  |  |  | $ 1,500 |  |
Future Office Building [Member] | 325 Main Street [Member] |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 400,000 |  |  |  |  |  |  |
Square Footage Of Signed Lease | ft² | 362,000 |  |  |  |  |  |  |
Term of Lease Signed (in years) | yr | 15 |  |  |  |  |  |  |
Future Retail Building [Member] | 325 Main Street [Member] |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 38,000 |  |  |  |  |  |  |
Subsequent Event [Member] | One Tower Center [Member] |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  | 410,000 |
Gross Sale Price |  |  |  |  |  |  | $ 38,000 |

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