Mortgage Notes Payable, Credit Facilities and Senior Notes (Carrying Value of Debt) (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019	Dec. 20, 2019	Nov. 10, 2017	Jul. 26, 2016	Dec. 03, 2014	Dec. 09, 2013
Debt Instrument [Line Items]
Credit facility	$ 100,544,000	$ 80,743,000
Senior Notes	943,267,000	942,850,000
Term Loan Agreement
Debt Instrument [Line Items]
Principal amount	300,000,000	300,000,000	$ 300,000,000.0
Net unamortized deferred financing costs	(1,567,000)	(1,670,000)
Term loan	298,433,000	298,330,000
Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility	103,500,000	84,000,000
Net unamortized deferred financing costs	(2,956,000)	(3,257,000)
Credit facility	100,544,000	80,743,000
Senior Notes | Senior Notes Due 2027
Debt Instrument [Line Items]
Principal amount	250,000,000	250,000,000		$ 250,000,000.0
Net unamortized deferred financing costs	(1,270,000)	(998,000)
Senior Notes	248,730,000	249,002,000
Senior Notes | Senior Notes Due 2026
Debt Instrument [Line Items]
Principal amount	200,000,000	200,000,000			$ 200,000,000.0
Net unamortized deferred financing costs	(462,000)	(191,000)
Senior Notes	199,538,000	199,809,000
Senior Notes | Senior Notes Due 2024
Debt Instrument [Line Items]
Principal amount	250,000,000	250,000,000				$ 250,000,000.0
Unamortized debt discount	(1,647,000)	(1,912,000)
Net unamortized deferred financing costs	(928,000)	(1,094,000)
Senior Notes	247,425,000	246,994,000
Senior Notes | Senior Notes Due 2023
Debt Instrument [Line Items]
Principal amount	250,000,000	250,000,000					$ 250,000,000.0
Unamortized debt discount	(1,582,000)	(1,915,000)
Net unamortized deferred financing costs	(844,000)	(1,040,000)
Senior Notes	$ 247,574,000	$ 247,045,000