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Interest Rate Swap Agreements (Details Textual) (USD $)
In Thousands, unless otherwise specified
 6 Months Ended
Jun. 30, 2012
Agreement
Jun. 30, 2011
Derivative [Line Items]    
Nominal amount of interest rate swap $ 650,000  
Interest Rate Swap Agreements (Textual) [Abstract]    
Number of interest rate swap agreements 5  
Changes in fair values are reclassified from accumulated other comprehensive loss into earnings 6,263 7,945
Amortized interest expenses related to a previously terminated interest rate swap agreement 1,976 2,259
Interest expense on interest rate swap agreement will amortize over the next twelve months 494  
Interest Rate Swap One Agreements [Member]
    
Derivative [Line Items]    
Nominal amount of interest rate swap 125,000  
Interest Rate Swap One Agreements [Member] | Nondesignated [Member]
    
Derivative [Line Items]    
Portion of interest rate swap not designated as hedge 18,368  
Interest Rate Swap Two Agreements [Member]
    
Derivative [Line Items]    
Nominal amount of interest rate swap 75,000  
Interest Rate Swap Two Agreements [Member] | Nondesignated [Member]
    
Derivative [Line Items]    
Portion of interest rate swap not designated as hedge 11,017  
Interest Rate Swap Three Agreements [Member]
    
Derivative [Line Items]    
Nominal amount of interest rate swap 175,000  
Interest Rate Swap Four Agreements [Member]
    
Derivative [Line Items]    
Nominal amount of interest rate swap 175,000  
Interest Rate Swap Five Agreements [Member]
    
Derivative [Line Items]    
Nominal amount of interest rate swap $ 100,000