Lessee leases (narrative)(Details) $ in Thousands			12 Months Ended
	Jul. 01, 2021 USD ($)	May 19, 2021 USD ($) ft²	Dec. 31, 2021 USD ($)		Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Number of Properties Subject to Ground Leases			4		4	4
Operating Lease, Payments			$ 23,599		$ (1,533)	$ (1,616)
Right of use assets - operating leases			169,778	[1]	146,406
Present value of lease payments			204,561		201,713
Operating lease costs			$ 13,151		$ 13,948	$ 14,573
Property Subject to Operating Lease
Lessee, Lease, Description [Line Items]
Number of Properties Subject to Ground Leases			4		4	4
Sumner Square
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Description		for an additional 15 years. Prior to the amendment, the ground lease was scheduled to expire on August 10, 2066. The ground lease will now expire on August 9, 2081.
Operating Lease, Payments	$ 23,000
Lessee Operating Lease Liability Payments Due, Next Five Years		$ 4,000
Lessee, Operating Lease, Discount Rate		3.95%
Operating Lease, Liability		$ 26,700
Right of use assets - operating leases		27,100
Present value of lease payments		$ 26,670	$ 3,400
Net Rentable Area (in sf) | ft²		210,000
Operating lease costs			$ 700		$ 1,400	$ 1,500

[1] See Note 4.