Derivatives (Details)		3 Months Ended
		Mar. 31, 2017 USD ($) derivative	Mar. 31, 2016 USD ($)	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		$ 8,452,000
Interest expense		26,614,000	$ 23,586,000
Interest rate hedges gain (loss) to be reclassified into interest expense over next 12 months		1,664,000
Treasury Lock
Derivative [Line Items]
Number of interest rate derivatives terminated | instrument								2	2
Aggregate Notional Amount								$ 150,000,000	$ 100,000,000
Liability Fair Value When Terminated								5,300,000	3,260,000
Fair value deferred in other comprehensive income	[1]							$ 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		$ 125,000,000		$ 180,000,000	$ 300,000,000	$ 225,000,000	$ 240,000,000
Liability Fair Value When Terminated				(13,352,000)	13,369,000	6,312,000	3,156,000
Fair value deferred in other comprehensive income	[1]			$ (13,345,000)	$ 13,369,000	$ 6,312,000	$ 3,141,000
Number of interest rate derivatives entered into | derivative		1
Reclassification out of Accumulated Other Comprehensive Income
Derivative [Line Items]
Interest expense		$ 447,000	$ 695,000
Designated as Hedging Instrument | Forward Swap
Derivative [Line Items]
Liability Fair Value When Terminated		$ 554,000

[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.