Derivatives (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) instrument	Sep. 30, 2017 USD ($) instrument	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	Jun. 30, 2011 USD ($) instrument
Derivative [Line Items]
Fair value of interest rate hedges recognized in other comprehensive income	$ 8,994,000		$ 8,994,000
Interest expense	27,194,000	$ 28,204,000	80,906,000	$ 82,092,000
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months, net	$ (1,896,000)		(1,896,000)
Reclassification out of Accumulated Other Comprehensive Income
Derivative [Line Items]
Interest expense			$ 1,581,000	$ 1,394,000
Treasury Lock
Derivative [Line Items]
Number of interest rate derivatives terminated | instrument									2
Aggregate Notional Amount									$ 150,000,000
Liability (Asset) Fair Value When Terminated									5,300,000
Fair Value Deferred In Other Comprehensive Income									$ 5,218,000
Forward Swap
Derivative [Line Items]
Number of interest rate derivatives terminated | instrument	2	2	2	2	2	4	3	4
Aggregate Notional Amount	$ 250,000,000	$ 250,000,000	$ 250,000,000	$ 250,000,000	$ 180,000,000	$ 300,000,000	$ 225,000,000	$ 240,000,000
Liability (Asset) Fair Value When Terminated		7,690,000		7,690,000		13,369,000	6,312,000	3,156,000
Liability (Asset) Fair Value When Terminated	(4,080,000)		(4,080,000)		(13,352,000)
Fair Value Deferred In Other Comprehensive Income	$ (4,080,000)	$ 7,688,000	$ (4,080,000)	$ 7,688,000	$ (13,345,000)	$ 13,369,000	$ 6,312,000	$ 3,141,000