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Background and Significant Accounting Policies (Details3) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Summary of fair value and carrying value of debt    
Carrying Amount $ 1,773,039 $ 1,897,147
Fair Value 1,781,000 1,909,000
Fixed Rate Debt [Member]
   
Summary of fair value and carrying value of debt    
Carrying Amount 794,039 775,810
Fair Value 792,000 808,000
Floating Rate Debt [Member]
   
Summary of fair value and carrying value of debt    
Carrying Amount 979,000 1,121,337
Fair Value $ 989,000 $ 1,101,000