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Real Estate Narrative (Details) $ in Thousands |  |  |  |  | 2 Months Ended | 3 Months Ended | 5 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
May 24, 2018 USD ($) ft² | Feb. 23, 2018 ft² | Jan. 09, 2018 USD ($) ft² | Jan. 08, 2018 USD ($) | May 23, 2018 USD ($) | Jun. 30, 2018 USD ($) | Jun. 30, 2017 USD ($) | May 23, 2018 USD ($) | Jun. 30, 2018 USD ($) | Jun. 30, 2017 USD ($) | Jun. 20, 2018 ft² apartments | Jun. 07, 2018 ft² apartments | Jan. 24, 2018 ft²
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  | $ 141,249 | $ 17,049 |  |  |
Gains on sales of real estate |  |  |  |  |  | $ 18,292 | $ 3,767 |  | 114,689 | 3,900 |  |  |
17Fifty Presidents Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  | 276,000
Signature at Reston [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  | 518,000 |
Number of apartment units | apartments |  |  |  |  |  |  |  |  |  |  |  | 508 |
Reston Gateway [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  | 1,100,000.0 |  |  |  |  |  |  |  |  |  |  |
Square Footage Of Signed Lease | ft² |  | 850,000 |  |  |  |  |  |  |  |  |  |  |
Proto Kendall Square [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  | 167,000 |  |
Number of apartment units | apartments |  |  |  |  |  |  |  |  |  |  | 280 |  |
Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  | 141,249 | 17,049 |  |  |
Gains on sales of real estate |  |  |  |  |  | $ 18,770 | 4,344 |  | $ 117,677 | 4,477 |  |  |
500 E Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 262,000 |  |  |  |  |  |  |  |  |  |
Net Contractual Sales Price |  |  | $ 118,600 |  |  |  |  |  |  |  |  |  |
Net income (loss) |  |  |  | $ 100 |  |  | 1,600 |  |  | 3,200 |  |  |
Proceeds from sales of real estate |  |  | 116,100 |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  | 96,400 |  |  |  |  |  |  |  |  |  |
500 E Street [Member] | Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  | $ 98,900 |  |  |  |  |  |  |  |  |  |
91 Hartwell Avenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 119,000 |  |  |  |  |  |  |  |  |  |  |  |
Gross Sale Price | $ 22,200 |  |  |  |  |  |  |  |  |  |  |  |
Net income (loss) |  |  |  |  | $ 100 |  | $ 100 | $ 300 |  | $ 300 |  |  |
Proceeds from sales of real estate | 21,700 |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate | 15,500 |  |  |  |  |  |  |  |  |  |  |  |
91 Hartwell Avenue [Member] | Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate | $ 15,900 |  |  |  |  |  |  |  |  |  |  |  |

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