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Derivatives (Details)
 3 Months Ended
Mar. 31, 2018
USD ($)
instrument
Mar. 31, 2017
USD ($)
Dec. 31, 2017
instrument
Sep. 30, 2017
USD ($)
instrument
Dec. 31, 2016
USD ($)
Oct. 31, 2015
USD ($)
instrument
May 31, 2014
USD ($)
instrument
Apr. 30, 2013
USD ($)
instrument
Jun. 30, 2011
USD ($)
instrument
Derivative [Line Items]                  
Fair value of interest rate hedges recognized in other comprehensive income $ 14,130,000                
Interest expense 26,602,000 $ 26,614,000              
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months, net (2,165,000)                
Gain (loss) on cash flow hedge ineffectiveness, net 0                
Treasury Lock                  
Derivative [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                  
Derivative [Line Items]                  
Number of interest rate derivatives terminated | instrument     2 2   4 3 4  
Aggregate Notional Amount 250,000,000     $ 250,000,000 $ 180,000,000 $ 300,000,000 $ 225,000,000 $ 240,000,000  
Liability (Asset) Fair Value When Terminated $ 2,164,000     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       $ 7,688,000 $ (13,345,000) $ 13,369,000 $ 6,312,000 $ 3,141,000  
Number of interest rate derivatives entered into | instrument 2                
Reclassification out of Accumulated Other Comprehensive Income                  
Derivative [Line Items]                  
Interest expense $ 525,000 $ 447,000