Mortgage Loans Payable and Unsecured Credit Facilities (Narrative) (Details)			12 Months Ended
	Jul. 24, 2018 USD ($)	Sep. 08, 2017 USD ($) Loan	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Apr. 26, 2016 USD ($)
Line Of Credit Facility [Line Items]
Payments of early extinguishment of debt costs			$ 4,829,000	$ 210,000	$ 2,623,000
Number of amended and restated existing unsecured term loans | Loan		4
Other assets and deferred charges, net			40,642,000	35,350,000
Approximate amount of accumulated other comprehensive loss to be reclassified into earnings			2,200,000
Interest Rate Swap [Member]
Line Of Credit Facility [Line Items]
Interest rate swaps liability			1,600,000
Derivative, notional amount			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			$ 8,900,000
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			$ 132,200,000
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 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%
Term Loan Maturing April 26, 2023 [Member] | Unsecured credit facility [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate			1.65%
Maturity date			Apr. 26, 2023
Term loan amount						$ 100,000,000
Term Loan Maturing April 26, 2023 [Member] | Unsecured credit facility [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate			1.65%
Term Loan Maturing April 26, 2023 [Member] | 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