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Real Estate (Narrative) (Details) $ / shares in Units, $ in Thousands |  |  |  |  | 2 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 02, 2021 USD ($) ft² a Buildings | Jun. 25, 2021 USD ($) ft² $ / shares | Jun. 02, 2021 USD ($) ft² Buildings | Apr. 19, 2021 USD ($) a ft² | Sep. 30, 2021 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Apr. 16, 2021 ft² | Feb. 25, 2021 ft² | Feb. 01, 2021 ft² | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  | $ 730,056 | $ 693,268 |  | $ 2,157,558 | $ 2,100,597 |  |  |  |  |
Cash and Cash Equivalents, at Carrying Value |  |  |  |  | $ 1,002,728 | 1,002,728 | 1,714,783 | $ 1,002,728 | 1,002,728 | 1,714,783 |  |  |  | $ 1,668,742 | $ 644,950
Cash held in escrows |  |  |  |  | 79,193 | 79,193 | 50,006 | 79,193 | 79,193 | 50,006 |  |  |  | 50,587 | 46,936
11251 Roger Bacon Drive |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Leased percentage |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  | 65,000 |  |  |  |  |  |  |  |  |  |  |
Area of Land | a |  |  |  | 2.6 |  |  |  |  |  |  |  |  |  |  |
11251 Roger Bacon Drive | Scenario, Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Real Estate |  |  |  | $ 5,600 |  |  |  |  |  |  |  |  |  |  |
153 & 211 Second Avenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Real Estate |  |  | $ 100,200 |  |  |  |  |  |  |  |  |  |  |  |
Number of real estate properties | Buildings |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |
Leased percentage |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 137,000 |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  | 3,100 |  |  |  |  |  |  |
Operating Income (Loss) |  |  |  |  |  |  |  | (1,200) |  |  |  |  |  |  |
One Five Nine East 53rd Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  | 220,000 |  |
Noncontrolling Interest, Ownership Percentage by Parent |  |  |  |  |  |  |  |  |  |  |  |  | 55.00% |  |
Shady Grove Bio+Tech Campus |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Real Estate | $ 118,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 435,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of Land | a | 31 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  | 500 |  |  |  |  |  |  |  |  |  |
Operating Income (Loss) |  |  |  |  | (600) |  |  |  |  |  |  |  |  |  |
Number of buildings | Buildings | 7 |  |  |  |  |  |  |  |  |  |  |  |  |  |
360 Park South Avenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  | 450,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Cash held in escrows |  | $ 30,000 |  |  |  |  |  |  |  |  |  |  |  |  |
360 Park South Avenue | Scenario, Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncash or Part Noncash Acquisition, Value of Assets Acquired |  | 300,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Noncash or Part Noncash Acquisition, Debt Assumed |  | 202,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transaction, Value of Consideration Given |  | $ 98,000 |  |  |  |  |  |  |  |  |  |  |  |  |
360 Park South Avenue | Minimum [Member] | Scenario, Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Closing price of common stock (in dollars per share) | $ / shares |  | $ 111.00 |  |  |  |  |  |  |  |  |  |  |  |  |
3625 - 3635 Peterson Way |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  | 218,000 |  |  |  |
6595 Springfield Center Drive |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (Losses) on Sales of Investment Real Estate |  |  |  |  |  |  |  |  | 8,100 |  |  |  |  |  |
Construction in Progress | 180 CityPoint |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  | 329,000 |  |  |
Construction in Progress | 880 Winter Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  | 224,000 |  |  |
Construction in Progress | The Prudential Center Observatory |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  | 59,000 |  |  |
Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  | 730,056 | 693,268 |  | 2,157,558 | 2,100,597 |  |  |  |  |
Cash and Cash Equivalents, at Carrying Value |  |  |  |  | 1,002,728 | 1,002,728 | 1,714,783 | 1,002,728 | 1,002,728 | 1,714,783 |  |  |  | 1,668,742 | 644,950
Cash held in escrows |  |  |  |  | $ 79,193 | $ 79,193 | $ 50,006 | $ 79,193 | $ 79,193 | $ 50,006 |  |  |  | $ 50,587 | $ 46,936

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