Real Estate (Narrative) (Details) $ in Thousands			1 Months Ended	12 Months Ended
		Dec. 14, 2023 USD ($) ft² a buildings	Dec. 31, 2023 USD ($) ft² Real_Estate_Properties	Dec. 31, 2023 USD ($) ft² Real_Estate_Properties	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 13, 2023 USD ($)	Nov. 30, 2023 ft²	Nov. 13, 2023	Oct. 05, 2023 ft²	Sep. 26, 2023 ft²	Jul. 20, 2023 ft²	Jun. 01, 2023 ft²	Apr. 29, 2023 ft²	Jan. 30, 2023 ft²	Jan. 05, 2023 ft²
Real Estate Properties [Line Items]
Total revenue				$ 3,273,569	$ 3,108,581	$ 2,888,621
Net Income (Loss) Attributable to Parent				190,215	848,947	505,195
Depreciation and amortization				830,813	749,775	$ 717,336
Carying value of the Company's investment in unconsolidated joint ventures			$ 1,377,319	$ 1,377,319	$ 1,715,911
Santa Monica Business Park [Member]
Real Estate Properties [Line Items]
Ownership Percentage	[1]				55.00%
Carying value of the Company's investment in unconsolidated joint ventures	[2]				$ 164,735
Joint Venture Partner [Member] | Santa Monica Business Park [Member]
Real Estate Properties [Line Items]
Ownership Percentage		45.00%
Construction in Progress
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²			2,700,000	2,700,000
Number of real estate properties | Real_Estate_Properties			10	10
Santa Monica Business Park [Member]
Real Estate Properties [Line Items]
Land Subject to Ground Leases | a		47
Net Rentable Area (in sf) | ft²		1,200,000
Number of real estate properties | buildings		21
Lessee, Finance Lease, Remaining Lease Term		75 years
Payments to Acquire Investments		$ 38,000
Net Working Capital, Including Cash Acquired from Acquisition		20,000
Gain (loss) on sale / consolidation		$ 29,900
Total revenue			$ 3,900
Net Income (Loss) Attributable to Parent			$ (900)
Land subject to ground lease, percentage		70.00%
Santa Monica Business Park [Member] | Santa Monica Business Park [Member]
Real Estate Properties [Line Items]
Gain (loss) on sale / consolidation		$ 29,900
Santa Monica Business Park [Member] | Mortgages [Member]
Real Estate Properties [Line Items]
Debt		$ 300,000
Santa Monica Business Park [Member] | Company's Share [Member]
Real Estate Properties [Line Items]
Carying value of the Company's investment in unconsolidated joint ventures							$ 149,900
290 Binney | Construction in Progress
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²																566,000
Leased percentage																100.00%
300 Binney Street
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²															195,000
Depreciation and amortization				$ 11,000
300 Binney Street | Noncontrolling Interest [Member]
Real Estate Properties [Line Items]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners									45.00%
300 Binney Street | Construction in Progress
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²															236,000
Leased percentage															100.00%
2100 Pennsylvania Avenue
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²														476,000
View Boston
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²													63,000
140 Kendrick Street - Building A
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²												104,000
180 CityPoint [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²											329,000
103 CityPoint
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²										113,000
105 Carnegie Center
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²								70,000
105 Carnegie Center | Future construction
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²								73,000

[1] During the year ended December 31, 2023, the Company completed the acquisition of its joint venture partner’s 45% ownership interest in Santa Monica Business Park, as described below in this Note 6 and Note 3.

[2] Investments with deficit balances aggregating approximately $39.9 million and $85.4 million at December 31, 2023 and December 31, 2022, respectively, are included within Other Liabilities in the Company’s Consolidated Balance Sheets.