Derivatives (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) instrument	Dec. 31, 2016 USD ($) instrument	Dec. 31, 2015 USD ($)	Sep. 30, 2017 USD ($) instrument	Oct. 31, 2015 USD ($) instrument	May 31, 2014 USD ($) instrument	Apr. 30, 2013 USD ($) instrument	Jun. 30, 2011 USD ($) instrument
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of interest rate hedges recognized in other comprehensive income	$ 14,655,000
Interest expense	109,109,000	$ 96,352,000	$ 90,008,000
Interest rate hedges loss to be reclassified into interest expense over next 12 months	$ 2,139,000
Number of derivative financial instruments outstanding | instrument	0
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
Liability (Asset) Fair Value When Terminated								5,300,000
Fair Value Deferred In Other Comprehensive Income								$ 5,218,000
Forward Swap
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of interest rate derivatives terminated | instrument		2		2	4	3	4
Aggregate Notional Amount		$ 180,000,000		$ 250,000,000	$ 300,000,000	$ 225,000,000	$ 240,000,000
Liability (Asset) Fair Value When Terminated				7,690,000	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)		$ 7,688,000	$ 13,369,000	$ 6,312,000	$ 3,141,000
Reclassification out of Accumulated Other Comprehensive Income
Schedule of Trading Securities and Other Trading Assets [Line Items]
Interest expense	$ 1,932,000	$ 2,802,000	$ 1,902,000