Mortgage Notes Payable, Credit Facilities and Senior Notes (Carrying Value of Debt) (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018	Nov. 10, 2017	Jul. 26, 2016	Sep. 29, 2015	Dec. 03, 2014	Dec. 09, 2013
Debt Instrument [Line Items]
Credit facility	$ 90,335,000	$ 153,689,000
Senior Notes	942,496,000	941,449,000
Term Loan Agreement
Debt Instrument [Line Items]
Principal amount	300,000,000	300,000,000			$ 300,000,000.0
Net unamortized deferred financing costs	(736,000)	(924,000)
Term loan	299,264,000	299,076,000
Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility	92,000,000	156,000,000
Net unamortized deferred financing costs	(1,665,000)	(2,311,000)
Credit facility	90,335,000	153,689,000
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	(1,029,000)	(1,123,000)
Senior Notes	248,971,000	248,877,000
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	(198,000)	(219,000)
Senior Notes	199,802,000	199,781,000
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	(1,998,000)	(2,252,000)
Net unamortized deferred financing costs	(1,149,000)	(1,314,000)
Senior Notes	246,853,000	246,434,000
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	(2,023,000)	(2,339,000)
Net unamortized deferred financing costs	(1,107,000)	(1,304,000)
Senior Notes	$ 246,870,000	$ 246,357,000