Debt - 2020 Activity (Details)				3 Months Ended			9 Months Ended
	Apr. 29, 2020 USD ($)	Apr. 17, 2020 USD ($) extention	Mar. 25, 2020 USD ($)	Sep. 30, 2020 USD ($) extention	Mar. 31, 2020	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) extention	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Debt Instrument, Unused Borrowing Capacity, Amount				$ 497,000,000.0			$ 497,000,000.0
Line of Credit Facility, Accordion Feature, Increase Limit		$ 600,000,000.0
Debt Instrument, Covenant Compliance					The Company was in compliance with all financial and other covenants		The Company was in compliance with all financial and other covenants
Net book value of properties that are collateral for debt arrangements				$ 82,300,000			$ 82,300,000		$ 85,500,000
Unsecured Credit Facility
Debt Instrument [Line Items]
Number Of Extensions | extention				2			2
Debt Instrument Term Extension Period							6 months
Accounts Payable and Accrued Liabilities [Member]
Debt Instrument [Line Items]
Interest payable				$ 7,800,000		$ 6,300,000	$ 7,800,000	$ 6,300,000
Interest Expense [Member]
Debt Instrument [Line Items]
Amortization of deferred financing fees				750,000		673,000	2,172,000	1,909,000
Line of Credit Facility, Commitment Fee Amount				$ 315,000		$ 348,000	995,000	$ 1,118,000
Wells Fargo Bank, National Association CMBS Loan | Johnstown, NY, Q4 2012, One [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Amount	$ 1,000,000.0
Write off of Deferred Debt Issuance Cost							$ 100,000
$300 Million Unsecured Term Loan [Member]
Debt Instrument [Line Items]
Write off of Deferred Debt Issuance Cost		$ 700,000
Number Of Extensions | extention		2
Debt Instrument Term Extension Period							1 year
First Extention Fee		0.15%
Second Extention Fee		0.0020
Debt Issuance Costs, Gross		$ 2,100,000
Deferred Financing Accrued Extension Fee		1,100,000
Debt Instrument, Fee Amount		$ 35,000
$200 Million Unsecured Term Loan F [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of Debt			$ 100,000,000.0