Derivatives (Details)		6 Months Ended
		Jun. 30, 2016 USD ($) derivative	Jun. 30, 2015 USD ($)	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		$ 23,644,000
Interest expense		(1,402,000)	$ (832,000)
Interest rate hedges gain (loss) to be reclassified into interest expense over next 12 months		(2,933,000)
Interest Rate Swap
Derivative [Line Items]
Aggregate Notional Amount											$ 100,000,000
Liability Fair Value When Terminated											3,260,000
Fair Value Deferred In Other Comprehensive Income	[1]										$ 3,228,000
Number of derivatives | instrument											2
Treasury Lock
Derivative [Line Items]
Aggregate Notional Amount										$ 150,000,000
Liability Fair Value When Terminated										5,300,000
Fair Value Deferred In Other Comprehensive Income	[1]									$ 5,218,000
Number of derivatives | instrument										2
Forward Swap
Derivative [Line Items]
Aggregate Notional Amount		180,000,000		$ 300,000,000		$ 225,000,000		$ 240,000,000
Liability Fair Value When Terminated				13,369,000		6,312,000		3,156,000
Fair Value Deferred In Other Comprehensive Income		$ 312,000		$ 13,369,000	[1]	$ 6,312,000	[1]	$ 3,141,000	[1]
Number of derivatives		2		4		3		4

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