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Real Estate (Narrative) (Details) $ in Thousands |  |  |  |  |  |  |  |  |  | 3 Months Ended | 5 Months Ended | 9 Months Ended | 11 Months Ended | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 20, 2018 USD ($) a | Dec. 13, 2018 USD ($) ft² | Nov. 30, 2018 USD ($) ft² | Nov. 29, 2018 USD ($) ft² a yr | Sep. 27, 2018 USD ($) ft² | May 24, 2018 USD ($) ft² | Feb. 23, 2018 ft² Buildings | Jan. 09, 2018 USD ($) ft² | Jan. 08, 2018 USD ($) | Dec. 31, 2018 USD ($) ft² | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | May 23, 2018 USD ($) | Sep. 26, 2018 USD ($) | Nov. 30, 2018 USD ($) ft² | Dec. 31, 2018 USD ($) ft² | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 01, 2018 ft² | Nov. 09, 2018 ft² | Sep. 01, 2018 ft² apartments | Aug. 07, 2018 ft² | Jun. 07, 2018 ft² apartments | Jan. 24, 2018 ft² | Dec. 31, 2015 USD ($)
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash held in escrows |  |  |  |  |  |  |  |  |  | $ 95,832 |  |  |  | $ 70,602 | $ 70,602 |  |  |  |  |  | $ 95,832 | $ 70,602 | $ 63,174 |  |  |  |  |  |  | $ 73,790
Revenues |  |  |  |  |  |  |  |  |  | 705,157 | $ 686,284 | $ 664,484 | $ 661,151 | 655,229 | 657,712 | $ 656,907 | $ 632,228 |  |  |  | 2,717,076 | 2,602,076 | 2,550,820 |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 455,409 | 29,810 | 122,750 |  |  |  |  |  |  |
Impairment losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11,812 | 0 | 1,783 |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 182,356 | 7,663 | 80,606 |  |  |  |  |  |  |
Note receivable |  |  |  |  |  |  |  |  |  | 19,468 |  |  |  | 0 |  |  |  |  |  |  | 19,468 | 0 |  |  |  |  |  |  |  |
Present value of expected net minimum lease payments |  |  |  |  |  |  |  |  |  | $ 64,187 |  |  |  |  |  |  |  |  |  |  | $ 64,187 |  |  |  |  |  |  |  |  |
500 E Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Contractual Sales Price |  |  |  |  |  |  |  | $ 118,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  | 262,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating Income (Loss) |  |  |  |  |  |  |  |  | $ 100 |  |  |  |  |  |  |  |  |  |  |  |  | 6,500 | 7,600 |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  | $ 116,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  |  |  |  | 96,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
91 Hartwell Avenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  | 119,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300 |  |  |  | 500 | 400 |  |  |  |  |  |  |
Contractual Sales Price |  |  |  |  |  | $ 22,200 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  | 21,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  |  | 15,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Quorum Office Park |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  | 268,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 700 |  |  | 900 | 900 |  |  |  |  |  |  |
Contractual Sales Price |  |  |  |  | $ 35,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  | 34,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  | 7,900 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
6595 Springfield Center Drive [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  | 634,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contractual Sales Price |  | $ 98,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  | 97,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment losses |  | $ 8,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
1333 New Hampshire Avenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  | 315,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 315,000 |  |  |  |  |  |  |  |  |  |
Operating Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6,700 |  | 7,400 | 6,100 |  |  |  |  |  |  |
Contractual Sales Price |  |  | $ 142,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 142,000 |  |  |  |  |  |  |  |  |  |
Future payment by an anchor tenant |  |  | 5,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5,500 |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  | 133,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  | 44,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tower Oaks Land [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loans Receivable with Fixed Rates of Interest | 0.0192 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contractual Sales Price | $ 46,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate | 25,900 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate | 15,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Note receivable | $ 21,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of Land | a | 41 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
17Fifty Presidents Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 276,000 |
191 Spring Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 171,000 |  |  |  |  |
Salesforce Tower |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,421,000 |  |  |  |  |  |
Ownership Percentage by Parent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 95.00% |  |  |  |  |  |
Signature at Reston |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 518,000 |  |
Number of apartment units | apartments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 508 |  |
Reston Gateway Master Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  | 1,100,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Square Footage Of Signed Lease | ft² |  |  |  |  |  |  | 850,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of real estate properties | Buildings |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proto Kendall Square |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 167,000 |  |  |  |
Number of apartment units | apartments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 280 |  |  |  |
343 Madison Avenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 850,000 |  |  |
Ownership Percentage by Parent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55.00% |  |  |
Platform 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  | 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital Lease - fixed purchase price |  |  |  | $ 134,800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash held in escrows |  |  |  | $ 15,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ground Lease Term (in years) | yr |  |  |  | 65 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of Land | a |  |  |  | 5.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Present value of expected net minimum lease payments |  |  |  | $ 12,397 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2600 Tower Oaks Boulevard |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  | 179,000 |  |  |  |  |  |  |  |  |  |  | 179,000 |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  |  |  |  |  |  | $ 3,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash held in escrows |  |  |  |  |  |  |  |  |  | 95,832 |  |  |  | 70,602 |  |  |  |  |  |  | $ 95,832 | 70,602 | 63,174 |  |  |  |  |  |  | $ 73,790
Revenues |  |  |  |  |  |  |  |  |  | 705,157 | $ 686,284 | $ 664,484 | $ 661,151 | 655,229 | $ 657,712 | $ 656,907 | $ 632,228 |  |  |  | 2,717,076 | 2,602,076 | 2,550,820 |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 455,409 | 29,810 | 122,750 |  |  |  |  |  |  |
Impairment losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,181 | 0 | 1,783 |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 190,716 | 8,240 | $ 82,775 |  |  |  |  |  |  |
Note receivable |  |  |  |  |  |  |  |  |  | 19,468 |  |  |  | $ 0 |  |  |  |  |  |  | $ 19,468 | $ 0 |  |  |  |  |  |  |  |
Boston Properties Limited Partnership | 500 E Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  |  |  |  | $ 98,900 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Boston Properties Limited Partnership | 91 Hartwell Avenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  |  | $ 15,900 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Boston Properties Limited Partnership | Quorum Office Park |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  | $ 9,200 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Boston Properties Limited Partnership | 1333 New Hampshire Avenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  | $ 48,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Boston Properties Limited Partnership | 2600 Tower Oaks Boulevard |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  |  |  |  |  |  | $ 1,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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