Derivatives (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) instrument	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) instrument	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) instrument	Oct. 31, 2015 USD ($) instrument	May 31, 2014 USD ($) instrument	Apr. 30, 2013 USD ($) instrument	Jun. 30, 2011 USD ($) instrument	Sep. 30, 2007 USD ($) instrument
Derivative [Line Items]
Aggregate Notional Amount	$ 250,000,000		$ 250,000,000
Liability (Asset) Fair Value When Terminated	7,690,000		7,690,000
Fair value of interest rate hedges recognized in other comprehensive income	15,193,000		15,193,000
Interest expense	28,204,000	$ 24,257,000	82,092,000	$ 71,923,000
Interest rate hedges gain (loss) to be reclassified into interest expense over next 12 months	$ (2,131,000)		$ (2,131,000)
Treasury Lock
Derivative [Line Items]
Number of interest rate derivatives terminated | instrument									2	2
Aggregate Notional Amount									$ 150,000,000	$ 100,000,000
Liability (Asset) Fair Value When Terminated									5,300,000	3,260,000
Fair Value Deferred In Other Comprehensive Income									$ 5,218,000	$ 3,228,000
Forward Swap
Derivative [Line Items]
Number of interest rate derivatives terminated | instrument	2		2		2	4	3	4
Aggregate Notional Amount	$ 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					(13,352,000)
Fair Value Deferred In Other Comprehensive Income	$ 7,688,000		7,688,000		$ (13,345,000)	$ 13,369,000	$ 6,312,000	$ 3,141,000
Reclassification out of Accumulated Other Comprehensive Income
Derivative [Line Items]
Interest expense			$ 1,394,000	$ 2,122,000