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Real Estate (Narrative) (Details) |  |  |  |  | 3 Months Ended | 5 Months Ended | 8 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 01, 2017 ft² yr | Aug. 30, 2017 USD ($) a | Jun. 13, 2017 USD ($) ft² | Apr. 19, 2017 USD ($) a | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Jun. 13, 2017 USD ($) ft² | Dec. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Nov. 28, 2017 ft² | Nov. 17, 2017 ft² | Sep. 16, 2017 ft² | Aug. 24, 2017 ft² yr | Jun. 29, 2017 USD ($) yr apartments | May 27, 2017 ft² | May 15, 2017 USD ($) ft² | Apr. 06, 2017 ft²
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  | $ 655,229,000 | $ 657,712,000 | $ 656,907,000 | $ 632,228,000 | $ 636,061,000 | $ 625,228,000 | $ 623,546,000 | $ 665,985,000 |  |  | $ 2,602,076,000 | $ 2,550,820,000 | $ 2,490,821,000 |  |  |  |  |  |  |  |
Operating Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 908,110,000 | 825,784,000 | 849,365,000 |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 29,810,000 | 122,750,000 | 602,600,000 |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,663,000 | 80,606,000 | 375,895,000 |  |  |  |  |  |  |  |
Present value of expected net minimum lease payments |  |  |  |  | 51,596,000 |  |  |  |  |  |  |  |  | $ 51,596,000 | 51,596,000 |  |  |  |  |  |  |  |  |  |
30 Shattuck Road [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contractual Sales Price |  |  |  | $ 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  | 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  | $ 3,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of Land | a |  |  |  | 9.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
40 Shattuck Road [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  | 122,000 |  |  |  |  |  |  |  |  |  | 122,000 |  |  |  |  |  |  |  |  |  |  |  |
Operating Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  | $ (28,000) |  |  | (50,000) | 300,000 |  |  |  |  |  |  |  |
Contractual Sales Price |  |  | $ 12,000,000 |  |  |  |  |  |  |  |  |  | $ 12,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  | 11,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  | 28,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reston Eastgate [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contractual Sales Price |  | $ 14,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  | 13,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  | $ 2,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of Land | a |  | 21.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
145 Broadway |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 485,000
Reservoir Place North [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 73,000 |  |
103 Carnegie Center [Member] | 103 Carnegie Center [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate purchase price |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,800,000 |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 96,000 |
Revenues |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,700,000 |  |  |  |  |  |  |  |  |  |  |
Operating Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (300,000) |  |  |  |  |  |  |  |  |  |  |
6595 Springfield Center Drive (TSA Headquarters) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 634,000 |  |  |  |
Term of Lease Signed (in years) | yr |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15 |  |  |  |
888 Boylston Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 417,000 |  |  |  |  |
191 Spring Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 171,000 |  |  |  |  |  |
20 CityPoint |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 211,000 |  |  |  |  |  |  |
2100 Pennsylvania Avenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² | 469,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ground Lease Term (in years) | yr | 75 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term of Lease Signed (in years) | yr | 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Square Footage Of Signed Lease | ft² | 288,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
MacArthur Transit Center [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term of Lease Signed (in years) | yr |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 99 |  |  |
Number of apartment units | apartments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 402 |  |  |
Present value of expected net minimum lease payments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 28,962,000 |  |  |
Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  | $ 655,229,000 | $ 657,712,000 | $ 656,907,000 | $ 632,228,000 | $ 636,061,000 | $ 625,228,000 | $ 623,546,000 | $ 665,985,000 |  |  | 2,602,076,000 | 2,550,820,000 | 2,490,821,000 |  |  |  |  |  |  |  |
Operating Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 916,250,000 | 837,411,000 | 857,358,000 |  |  |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 29,810,000 | 122,750,000 | 602,600,000 |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,240,000 | $ 82,775,000 | $ 377,093,000 |  |  |  |  |  |  |  |
Boston Properties Limited Partnership | 40 Shattuck Road [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  | $ 600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Consolidated Properties [Member] | Salesforce Tower [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area | ft² | 1,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage by Parent | 95.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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