Mortgage Loans Payable and Credit Facility (Narrative) (Details) - USD ($)			3 Months Ended
	Apr. 01, 2019	Sep. 08, 2017	Mar. 31, 2019	Dec. 31, 2018
Line Of Credit Facility [Line Items]
Other assets and deferred charges, net			$ 50,743,000	$ 40,642,000
Approximate amount of accumulated other comprehensive loss to be reclassified into earnings			1,700,000
Interest Rate Swap [Member]
Line Of Credit Facility [Line Items]
Interest rate swaps liability			2,900,000
Derivative, notional amount			425,000,000	$ 425,000,000
Cash Flow Hedging, Count 3 [Member] | Interest Rate Swap [Member]
Line Of Credit Facility [Line Items]
Other assets and deferred charges, net			4,700,000
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]
Credit facility borrowing capacity			$ 105,200,000
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
Revolving Credit Facility [Member] | Unsecured credit facility [Member] | Subsequent Event [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate	0.14%
Revolving Credit Facility [Member] | Unsecured credit facility [Member] | Minimum [Member] | Subsequent Event [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate	0.10%
Revolving Credit Facility [Member] | Unsecured credit facility [Member] | Maximum [Member] | Subsequent Event [Member]
Line Of Credit Facility [Line Items]
Basis spread on borrowings variable rate	0.15%
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