Mortgage Loans Payable and Credit Facility (Narrative) (Details) - USD ($)	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Line Of Credit Facility [Line Items]
Other assets and deferred charges, net	$ 27,686,000	$ 29,506,000
Approximate amount of accumulated other comprehensive loss to be reclassified into earnings	1,500,000
Interest Rate Swap [Member]
Line Of Credit Facility [Line Items]
Fair value of interest rate swaps included in accounts payable and accrued liabilities	1,700,000
Cash Flow Hedging, Count 3 [Member] | Interest Rate Swap [Member]
Line Of Credit Facility [Line Items]
Other assets and deferred charges, net	2,700,000
Unsecured credit facility [Member]
Line Of Credit Facility [Line Items]
Credit facility borrowing capacity	310,000,000
Aggregate borrowing capacity including increase under accordion feature	750,000,000
Revolving Credit Facility [Member] | Unsecured credit facility [Member]
Line Of Credit Facility [Line Items]
Credit facility borrowing capacity	260,000,000
Remaining borrowing capacity	161,600,000
Term loan facility [Member] | Unsecured credit facility [Member]
Line Of Credit Facility [Line Items]
Credit facility borrowing capacity	$ 50,000,000