Derivatives (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) derivative	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) derivative	Jun. 30, 2016 USD ($)	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]
Fair value of interest rate hedges recognized in other comprehensive income	$ 7,997,000		$ 7,997,000
Interest expense	27,274,000	$ 24,081,000	53,888,000	$ 47,667,000
Interest rate hedges gain (loss) to be reclassified into interest expense over next 12 months	(1,578,000)		(1,578,000)
Net hedge ineffectiveness gain (loss)			0
Designated as Hedging Instrument
Derivative [Line Items]
Liability (Asset) Fair Value When Terminated	2,355,000		2,355,000
Liability (Asset) Fair Value When Terminated	(661,000)		(661,000)
Fair Value Deferred In Other Comprehensive Income	(1,694,000)		(1,694,000)
Cash flow hedge derivative instrument assets at fair value	(661,000)		(661,000)
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	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						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					(13,345,000)	$ 13,369,000	$ 6,312,000	$ 3,141,000
Number of interest rate derivatives entered into | derivative	2		2
Cash flow hedge derivative instrument assets at fair value					$ (13,352,000)
Reclassification out of Accumulated Other Comprehensive Income
Derivative [Line Items]
Interest expense			$ 903,000	$ 1,402,000