Derivatives (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) instrument	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2018 USD ($) instrument	Sep. 30, 2017 USD ($) instrument	Dec. 31, 2016 USD ($) instrument	Oct. 31, 2015 USD ($) instrument	May 31, 2014 USD ($) instrument	Apr. 30, 2013 USD ($) instrument
Debt and Equity Securities, FV-NI [Line Items]
Fair Value Deferred In Other Comprehensive Income	$ (5,524,000)
Remaining amount in other comprehensive income for effective portion of previous interest rate hedges	5,604,000
Interest expense	120,023,000	$ 115,847,000	$ 109,109,000
Interest rate hedges loss to be reclassified into interest expense over next 12 months	1,365,000
Reclassification out of Accumulated Other Comprehensive Income
Debt and Equity Securities, FV-NI [Line Items]
Interest expense	1,307,000	$ 3,664,000	$ 1,932,000
Forward Swap
Debt and Equity Securities, FV-NI [Line Items]
Number of interest rate derivatives terminated | instrument				2	2	2	4	3	4
Aggregate Notional Amount	$ 200,000,000			$ 250,000,000	$ 250,000,000	$ 180,000,000	$ 300,000,000	$ 225,000,000	$ 240,000,000
Liability Fair Value When Terminated					7,690,000		13,369,000	6,312,000	3,156,000
Asset Fair Value When Terminated				(4,080,000)		(13,352,000)
Fair Value Deferred In Other Comprehensive Income				$ (4,080,000)	$ 7,688,000	$ (13,345,000)	$ 13,369,000	$ 6,312,000	$ 3,141,000
Number of derivative financial instruments entered | instrument	3