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Real Estate (Narrative) (Details) $ in Thousands |  |  |  |  |  |  |  |  |  |  |  |  | 3 Months Ended | 4 Months Ended | 5 Months Ended | 7 Months Ended | 8 Months Ended | 10 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |  |
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Nov. 08, 2022 USD ($) ft² floor | Oct. 19, 2022 ft² Building | Sep. 16, 2022 USD ($) ft² | Sep. 15, 2022 USD ($) land_parcel | Aug. 30, 2022 USD ($) ft² | Jul. 01, 2022 ft² Building | Jun. 15, 2022 USD ($) ft² Building | May 17, 2022 USD ($) ft² floor | Apr. 27, 2022 ft² Building | Apr. 07, 2022 USD ($) | Mar. 31, 2022 USD ($) ft² | Apr. 19, 2021 USD ($) a ft² | Mar. 30, 2022 USD ($) | Dec. 31, 2022 USD ($) ft² | Jun. 14, 2022 USD ($) | Dec. 31, 2022 USD ($) ft² | Aug. 29, 2022 USD ($) | Nov. 07, 2022 USD ($) | Dec. 31, 2022 USD ($) ft² | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 23, 2022 ft² | Dec. 01, 2022 ft² | Nov. 30, 2022 ft² | Sep. 12, 2022 ft² | Sep. 08, 2022 ft² | Jun. 29, 2022 ft² | May 13, 2022 ft² | Apr. 29, 2022 ft² | May 09, 2019 ft²
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unsecured term loan |  |  |  |  |  |  |  | $ 730,000 |  |  |  |  |  | $ 730,000 |  | $ 730,000 |  |  | $ 730,000 | $ 0 |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,108,581 | 2,888,621 | $ 2,765,686 |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,020,584 | 631,932 | 1,018,691 |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 834,770 | 179,887 | 519,303 |  |  |  |  |  |  |  |  |
Interest and Other Income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11,940 | 5,704 | 5,953 |  |  |  |  |  |  |  |  |
Other income - assignment fee |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6,624 | 0 | 0 |  |  |  |  |  |  |  |  |
Construction in Progress |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,200,000 |  | 3,200,000 |  |  | 3,200,000 |  |  |  |  |  |  |  |  |  |  |
Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unsecured term loan |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 730,000 |  | $ 730,000 |  |  | $ 730,000 | 0 |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,108,581 | 2,888,621 | 2,765,686 |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,032,122 | 641,771 | 1,038,739 |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 834,770 | 179,887 | 519,303 |  |  |  |  |  |  |  |  |
Interest and Other Income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11,940 | 5,704 | 5,953 |  |  |  |  |  |  |  |  |
Other income - assignment fee |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6,624 | 0 | 0 |  |  |  |  |  |  |  |  |
Madison Centre |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncash or Part Noncash Acquisition, Value of Assets Acquired |  |  |  |  |  |  |  | $ 724,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  | 755,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of floors | floor |  |  |  |  |  |  |  | 37 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 32,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |
125 Broadway |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Real Estate |  |  | $ 592,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 271,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | 11,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
195 West Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  | 63,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  |  |  |  |  | $ 400 |  |  |  |  |  |  | 500 | (800) |  |  |  |  |  |  |  |  |
Contractual Sales Price |  |  |  |  |  |  |  |  |  |  | $ 37,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  | 35,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (Losses) on Sales of Investment Real Estate |  |  |  |  |  |  |  |  |  |  | 22,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
195 West Street | Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (Losses) on Sales of Investment Real Estate |  |  |  |  |  |  |  |  |  |  | $ 23,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
11251 Roger Bacon Drive |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  | 65,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Asset Acquisition, Price of Acquisition, Expected |  |  |  |  |  |  |  |  |  |  |  | $ 5,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest and Other Income |  |  |  |  |  |  |  |  |  | $ 6,900 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other income - assignment fee |  |  |  |  |  |  |  |  |  | $ 6,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of Land | a |  |  |  |  |  |  |  |  |  |  |  | 2.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Virginia 95 Office Park |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  | 733,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,300 |  |  |  |  | 7,000 | 7,800 |  |  |  |  |  |  |  |  |
Contractual Sales Price |  |  |  |  |  |  | $ 127,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  | 121,900 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (Losses) on Sales of Investment Real Estate |  |  |  |  |  |  | $ 96,200 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of buildings | Building |  |  |  |  |  |  | 11 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Virginia 95 Office Park | Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (Losses) on Sales of Investment Real Estate |  |  |  |  |  |  | $ 99,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
601 Massachusetts Avenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  | 479,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 14,900 |  |  | 22,400 | 21,800 |  |  |  |  |  |  |  |  |
Contractual Sales Price |  |  |  |  | $ 531,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  | 512,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (Losses) on Sales of Investment Real Estate |  |  |  |  | 237,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
601 Massachusetts Avenue | Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (Losses) on Sales of Investment Real Estate |  |  |  |  | $ 237,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loudoun County Land |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contractual Sales Price |  |  |  | $ 27,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  | 25,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (Losses) on Sales of Investment Real Estate |  |  |  | $ 24,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Parcels of undeveloped land | land_parcel |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
The Avant at Reston Town Center |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,100 |  | $ 4,000 | $ 4,000 |  |  |  |  |  |  |  |  |
The Avant at Reston Town Center | Residential Building |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 329,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of floors | floor | 15 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contractual Sales Price | $ 141,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate | 139,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (Losses) on Sales of Investment Real Estate | $ 55,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Units in Real Estate Property | 359 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
The Avant Retail | Retail Site [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 26,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
290 Binney Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  | 566,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lessor, Operating Lease, Lease Not yet Commenced, Description |  |  |  |  |  |  |  |  | 15-year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Kendall Center future development | Scenario, Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  | 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of buildings | Building |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Kendall Center future development | Scenario, Plan | Residential Building |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  | 400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2100 Pennsylvania Avenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 480,000 |
Reston Next Office Phase II |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 90,000 |  |
325 Main Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 414,000 |  |  | 115,000
140 Kendrick Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  | 388,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of buildings | Building |  |  |  |  |  | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
140 Kendrick - Building A |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  | 90,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of buildings | Building |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
140 Kendrick - Building A | Construction in Progress |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  | 104,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
300 Binney Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 195,000 |  |  |  |
300 Binney Street | Construction in Progress |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 240,000 |  |  |  |
760 Boylston Street [Member] | Construction in Progress |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 118,000 |  |  |  |  |
Reston Next |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  | 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of buildings | Building |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
105 Carnegie Center |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 70,000 |  |  |  |  |  |
105 Carnegie Center | Construction in Progress |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 73,000 |  |  |  |  |  |
2096 Gaither Road |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50,000 |  |  |  |  |  |  |
880 Winter Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 244,000 |  |  |  |  |  |  |  |

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