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Real Estate Narrative (Details) $ in Thousands |  |  |  |  |  | 3 Months Ended | 5 Months Ended | 9 Months Ended |  |  |  |  |
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Sep. 27, 2018 USD ($) ft² | May 24, 2018 USD ($) ft² | Feb. 23, 2018 ft² Buildings | Jan. 09, 2018 USD ($) ft² | Jan. 08, 2018 USD ($) | Sep. 30, 2018 USD ($) ft² | Sep. 26, 2018 USD ($) | Sep. 30, 2017 USD ($) | May 23, 2018 USD ($) | Sep. 30, 2018 USD ($) ft² | Sep. 26, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 01, 2018 ft² apartments | Aug. 30, 2018 USD ($) ft² | Aug. 07, 2018 ft² | Jun. 07, 2018 ft² apartments | Jan. 24, 2018 ft²
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  | $ 175,577 |  | $ 29,810 |  |  |  |  |
Gains on sales of real estate |  |  |  |  |  | $ 7,863 |  | $ 2,891 |  | 122,552 |  | 6,791 |  |  |  |  |
Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  | 175,577 |  | 29,810 |  |  |  |  |
Gains on sales of real estate |  |  |  |  |  | $ 9,154 |  | 2,891 |  | $ 126,831 |  | 7,368 |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of real estate properties | Buildings |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proto Kendall Square [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  | 167,000 |  |  |  |
Number of apartment units | apartments |  |  |  |  |  |  |  |  |  |  |  |  | 280 |  |  |  |
343 Madison Avenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 850,000 |  |
Ownership Percentage by the Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55.00% |  |
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 |  |  | 1,700 |  |  |  | 4,900 |  |  |  |  |
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 | $ 300 |  |  | 400 |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
1333 New Hampshire Avenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  | 316,000 |  |  |
Gross Sale Price |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 136,500 |  |  |
Quorum Office Park [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 268,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sale Price | $ 35,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income (loss) |  |  |  |  |  |  | $ 200 | $ 300 |  |  | $ 700 | $ 700 |  |  |  |  |
Proceeds from sales of real estate | 34,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate | 7,900 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Quorum Office Park [Member] | Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate | $ 9,200 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carnegie Center purchase option [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  | 1,700,000.0 |  |  |  | 1,700,000.0 |  |  |  |  |  |  |
Aggregate purchase price |  |  |  |  |  | $ 42,400 |  |  |  | $ 42,400 |  |  |  |  |  |  |
Deferred purchase price |  |  |  |  |  | $ 9,200 |  |  |  | $ 9,200 |  |  |  |  |  |  |

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