Mortgage Loans Payable and Credit Facility (Narrative) (Details) - USD ($)		3 Months Ended
	Sep. 08, 2017	Mar. 31, 2018	Dec. 31, 2017
Line Of Credit Facility [Line Items]
Other assets and deferred charges, net		$ 42,028,000	$ 35,350,000
Approximate amount of accumulated other comprehensive loss to be reclassified into earnings		1,100,000
Interest Rate Swap [Member]
Line Of Credit Facility [Line Items]
Derivative, notional amount		350,000,000	$ 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		$ 11,700,000
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		$ 98,700,000
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