Derivatives (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) instrument	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) instrument	Jun. 30, 2017 USD ($)	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	$ 13,599,000		$ 13,599,000
Interest expense	27,110,000	$ 27,274,000	53,712,000	$ 53,888,000
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months, net	(2,192,000)		(2,192,000)
Gain (loss) on cash flow hedge ineffectiveness, net			0
Reclassification out of Accumulated Other Comprehensive Income
Derivative [Line Items]
Interest expense			1,056,000	$ 903,000
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	4	3	4
Aggregate Notional Amount	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		13,369,000	6,312,000	3,156,000
Liability (Asset) Fair Value When Terminated	$ (1,095,000)		$ (1,095,000)			(13,352,000)
Fair Value Deferred In Other Comprehensive Income					$ 7,688,000	$ (13,345,000)	$ 13,369,000	$ 6,312,000	$ 3,141,000
Number of interest rate derivatives entered into | instrument	2		2