Interest Rate Derivatives (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income      
Amount of losses recognized in accumulated other comprehensive loss (“AOCL”) (effective portion) $ (3,474,000) $ (2,123,000)  
Interest Expense [Member]      
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income      
Amount of losses reclassified from AOCL into interest expense (effective portion) 773,000 695,000  
Interest rate swaps      
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income      
Approximate loss amount to be reclassified from AOCI to interest expense over the next 12 months (3,200,000)    
Interest rate derivatives in liability position, fair value 4,300,000    
Termination value to settle obligations under interest rate derivative agreements 4,500,000    
Interest rate swaps | Prepaid expenses and other current assets      
Fair value of interest rate derivatives and balance sheet classification      
Interest rate derivatives 0   274,000
Interest rate swaps | Interest rate derivatives      
Fair value of interest rate derivatives and balance sheet classification      
Fair value of Interest rate swaps classified as interest rate derivatives (4,282,000)   (1,855,000)
Designated      
Fair values of interest rate swap derivatives      
Fair value of interest rate swaps (4,282,000)   (1,581,000)
Designated | Interest rate swap, effective date January 3, 2012, swap three      
Fair values of interest rate swap derivatives      
Notional Amount 100,000,000    
Fixed rate (as a percent) 0.832%    
Fair value of interest rate swaps (267,000)   (407,000)
Designated | Interest rate swap, effective date January 3, 2012, swap four      
Fair values of interest rate swap derivatives      
Notional Amount 100,000,000    
Fixed rate (as a percent) 0.832%    
Fair value of interest rate swaps (267,000)   (407,000)
Designated | Interest rate swap, effective date November 2, 2010      
Fair values of interest rate swap derivatives      
Notional Amount 36,668,000    
Fixed rate (as a percent) 3.83%    
Fair value of interest rate swaps (291,000)   (400,000)
Notional amount of interest rate derivatives after scheduled amortization 36,200,000    
Designated | Interest rate swap, effective date November 2, 2010 | London Interbank Offered Rate (LIBOR) [Member]      
Fair values of interest rate swap derivatives      
Derivative, basis spread on variable rate 2.25%    
Designated | Interest rate swap, effective date September 2, 2014, swap one      
Fair values of interest rate swap derivatives      
Notional Amount 100,000,000    
Fixed rate (as a percent) 0.8055%    
Fair value of interest rate swaps (455,000)   (317,000)
Designated | Interest rate swap, effective date September 2, 2014, swap two      
Fair values of interest rate swap derivatives      
Notional Amount 100,000,000    
Fixed rate (as a percent) 0.81%    
Fair value of interest rate swaps (461,000)   (324,000)
Designated | Interest rate swap, effective date September 1, 2015, swap one      
Fair values of interest rate swap derivatives      
Notional Amount 100,000,000    
Fixed rate (as a percent) 1.673%    
Fair value of interest rate swaps (1,161,000)   239,000
Designated | Interest rate swap, effective date September 1, 2015, swap two      
Fair values of interest rate swap derivatives      
Notional Amount 100,000,000    
Fixed rate (as a percent) 1.73%    
Fair value of interest rate swaps $ (1,380,000)   $ 35,000