Mortgage Loans Payable and Credit Facility (Narrative) (Details) - USD ($)		3 Months Ended	9 Months Ended
	Sep. 08, 2017	Sep. 30, 2018	Sep. 30, 2018	Jul. 24, 2018	Dec. 31, 2017
Line Of Credit Facility [Line Items]
Payments of early extinguishment of debt costs		$ 4,829,000	$ 4,829,000
Other assets and deferred charges, net		50,645,000	50,645,000		$ 35,350,000
Approximate amount of accumulated other comprehensive loss to be reclassified into earnings			2,600,000
Interest Rate Swap [Member]
Line Of Credit Facility [Line Items]
Derivative, notional amount		425,000,000	425,000,000		$ 350,000,000
Cash Flow Hedging, Count 3 [Member] | Interest Rate Swap [Member]
Line Of Credit Facility [Line Items]
Other assets and deferred charges, net		15,800,000	$ 15,800,000
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		$ 135,100,000	$ 135,100,000
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 24, 2025 [Member] | Unsecured credit facility [Member]
Line Of Credit Facility [Line Items]
Loan maturity date			Jul. 24, 2018
Term loan amount				$ 75,000,000
Term Loan Maturing July 24, 2025 [Member] | Unsecured credit facility [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate			1.70%
Term Loan Maturing July 24, 2025 [Member] | Unsecured credit facility [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate			2.25%