Mortgage Loans Payable and Credit Facility (Narrative) (Details) - USD ($)		6 Months Ended
	Sep. 08, 2017	Jun. 30, 2018	Jul. 24, 2018	Dec. 31, 2017
Line Of Credit Facility [Line Items]
Other assets and deferred charges, net		$ 41,258,000		$ 35,350,000
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		100,000
Derivative, notional amount		400,000,000		$ 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		$ 13,800,000
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		$ 109,200,000
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
Debt instrument, description		Proceeds from the term loan can be drawn at any time from closing until October 24, 2018, and are expected to be used primarily to repay mortgages maturing through November 2022.
Mortgages maturity period		2022-11
Effective interest rate		4.60%
Term Loan Maturing July 24, 2025 [Member] | Unsecured credit facility [Member] | Subsequent Event [Member]
Line Of Credit Facility [Line Items]
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%