Derivatives (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) instrument	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) instrument	Sep. 30, 2018 USD ($) instrument	Sep. 30, 2017 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]
Remaining amount in other comprehensive income for effective portion of previous interest rate hedges	$ 6,911,000
Interest expense	115,847,000	$ 109,109,000	$ 96,352,000
Interest rate hedges loss to be reclassified into interest expense over next 12 months	$ 1,306,000
Number of derivative financial instruments outstanding | instrument	0
Reclassification out of Accumulated Other Comprehensive Income
Debt and Equity Securities, FV-NI [Line Items]
Interest expense	$ 3,664,000	$ 1,932,000	$ 2,802,000
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			$ 180,000,000	$ 250,000,000	$ 250,000,000	$ 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			(13,352,000)	(4,080,000)
Fair Value Deferred In Other Comprehensive Income			$ (13,345,000)	$ (4,080,000)	$ 7,688,000	$ 13,369,000	$ 6,312,000	$ 3,141,000