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Subsequent Events (Details) $ in Thousands | Aug. 02, 2021 USD ($) ft² a Vehicles | Jul. 20, 2021 USD ($) ft² | Jul. 13, 2021 USD ($) ft² | Jun. 30, 2021 USD ($) | Jun. 29, 2021 USD ($) ft² | Jun. 07, 2021 USD ($) a ft² | Dec. 31, 2020 USD ($) | Jun. 30, 2020 USD ($) | Dec. 31, 2019 USD ($)
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Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Cash and Cash Equivalents, at Carrying Value |  |  |  | $ 557,307 |  |  | $ 1,668,742 | $ 1,691,047 | $ 644,950
Safeco Plaza |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  | 800,000 |  |  |  |
Cash and Cash Equivalents, at Carrying Value |  |  |  |  | $ 46,500 |  |  |  |
Shady Grove Bio+Tech Campus |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  | 435,000 |  |  |
Cash and Cash Equivalents, at Carrying Value |  |  |  |  |  | $ 10,000 |  |  |
Area of Land | a |  |  |  |  |  | 31 |  |  |
Subsequent Event [Member] | 181, 191 and 201 Spring Street |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Contractual Sales Price |  |  | $ 191,500 |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 333,000 |  |  |  |  |  |
Leased percentage |  |  | 100.00% |  |  |  |  |  |
Subsequent Event [Member] | Safeco Plaza |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  | 800,000 |  |  |  |  |  |  |
Payments to Acquire Real Estate |  | $ 465,000 |  |  |  |  |  |  |
Cash and Cash Equivalents, at Carrying Value |  | 51,500 |  |  |  |  |  |  |
Subsequent Event [Member] | Safeco Plaza | Company's Share [Member] |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Cash and Cash Equivalents, at Carrying Value |  | $ 26,300 |  |  |  |  |  |  |
Subsequent Event [Member] | Shady Grove Bio+Tech Campus |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 435,000 |  |  |  |  |  |  |  |
Payments to Acquire Real Estate | $ 116,500 |  |  |  |  |  |  |  |
Number of real estate properties | Vehicles | 7 |  |  |  |  |  |  |  |
Area of Land | a | 31 |  |  |  |  |  |  |  |

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