Derivatives (Details)		9 Months Ended
		Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	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		$ 12,323,000
Interest expense		1,256,000	$ 736,000
Interest rate hedges gain (loss) to be reclassified into interest expense over next 12 months		1,751,000
Interest Rate Swap
Derivative [Line Items]
Number of derivatives | instrument									2
Aggregate Notional Amount									$ 100,000,000
Fair Value When Terminated	[1]								3,260,000
Fair Value Deferred In Other Comprehensive Income	[2]								$ 3,228,000
Treasury Lock
Derivative [Line Items]
Number of derivatives | instrument								2
Aggregate Notional Amount								$ 150,000,000
Fair Value When Terminated	[1]							5,300,000
Fair Value Deferred In Other Comprehensive Income	[2]							$ 5,218,000
Forward Swap
Derivative [Line Items]
Number of derivatives | instrument				3		4
Aggregate Notional Amount		300,000,000		$ 225,000,000		$ 240,000,000
Fair Value When Terminated		$ 10,297,000		6,312,000	[1]	3,156,000	[1]
Fair Value Deferred In Other Comprehensive Income	[2]			$ 6,312,000		$ 3,141,000

[1] Liability

[2] The amount reported in accumulated other comprehensive income will be reclassified to interest expense as interest payments are made on the related notes payable.