Derivatives (Details)		12 Months Ended
		Dec. 31, 2016 USD ($) instrument	Dec. 31, 2015 USD ($)	Dec. 31, 2014 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
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of interest rate hedges recognized in other comprehensive income		$ 8,899,000
Interest expense		96,352,000	$ 90,008,000	$ 85,510,000
Interest rate hedges loss to be reclassified into interest expense over next 12 months		$ 1,746,000
Number of derivative financial instruments outstanding | instrument		0
Reclassification out of Accumulated Other Comprehensive Income
Schedule of Trading Securities and Other Trading Assets [Line Items]
Interest expense		$ 2,802,000	$ 1,902,000	$ 1,129,000
Interest Rate Swap
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of interest rate derivatives terminated | instrument									2
Treasury Locks
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of interest rate derivatives terminated | instrument								2
Aggregate Notional Amount								$ 150,000,000	$ 100,000,000
Liability (Asset) Fair Value When Terminated								5,300,000	3,260,000
Liability amount from termination of hedge agreement, comprehensive income	[1]							$ 5,218,000	$ 3,228,000
Forward Swap
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of interest rate derivatives terminated | instrument		2,000			4,000	3	4
Aggregate Notional Amount		$ 180,000,000			$ 300,000,000	$ 225,000,000	$ 240,000,000
Liability (Asset) Fair Value When Terminated		(13,352,000)			13,369,000	6,312,000	3,156,000
Liability amount from termination of hedge agreement, comprehensive income	[1]	$ (13,345,000)			$ 13,369,000	$ 6,312,000	$ 3,141,000

[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.