Mortgage Loans Payable and Unsecured Credit Facilities (Narrative) (Details) - USD ($)			12 Months Ended
	Jul. 24, 2018	Sep. 08, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Line Of Credit Facility [Line Items]
Payments of early extinguishment of debt costs				$ 4,829,000	$ 210,000
Other assets and deferred charges, net			$ 42,139,000	40,642,000
Approximate amount of accumulated other comprehensive loss to be reclassified into earnings			1,800,000
Interest Rate Swap [Member]
Line Of Credit Facility [Line Items]
Interest rate swaps liability			7,200,000
Derivative, notional amount			425,000,000	425,000,000
Cash Flow Hedging, Count 3 [Member] | Interest Rate Swap [Member]
Line Of Credit Facility [Line Items]
Other assets and deferred charges, net			$ 100,000
Unsecured credit facility [Member]
Line Of Credit Facility [Line Items]
Credit facility borrowing capacity		$ 300,000,000
Aggregate borrowing capacity including increase under accordion feature		$ 750,000,000
Revolving Credit Facility [Member]
Line Of Credit Facility [Line Items]
Line of credit facility extension allowed period		1 year	1 year
Basis spread on borrowings variable rate			1.35%
Revolving Credit Facility [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate		1.35%
Revolving Credit Facility [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate		1.95%
Revolving Credit Facility [Member] | Unsecured credit facility [Member]
Line Of Credit Facility [Line Items]
Credit facility borrowing capacity		$ 250,000,000
Credit facility expiration date		Sep. 08, 2021
Remaining borrowing capacity			$ 95,600,000
Term loan facility [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate			1.30%
Term loan facility [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate		1.30%
Term loan facility [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate		1.90%
Term loan facility [Member] | Unsecured credit facility [Member]
Line Of Credit Facility [Line Items]
Credit facility borrowing capacity		$ 50,000,000
Credit facility expiration date		Sep. 08, 2022
Term Loan Maturing July Twenty Four Two Thousand Twenty Five | Unsecured credit facility [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate			1.70%
Maturity date			Jul. 24, 2018
Term loan amount	$ 75,000,000
Term Loan Maturing July Twenty Four Two Thousand Twenty Five | Unsecured credit facility [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate	1.70%
Term Loan Maturing July Twenty Four Two Thousand Twenty Five | Unsecured credit facility [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate	2.25%
Continuing Operations [Member]
Line Of Credit Facility [Line Items]
Payments of early extinguishment of debt costs				$ 4,800,000	$ 100,000