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Debt (Details 2) (USD $)
In Thousands
Sep. 30, 2011
Dec. 31, 2010
Carrying amount and estimated fair value of debt  
Debt, net$ 2,420,073$ 2,323,681
Carrying Amount
  
Carrying amount and estimated fair value of debt  
Fixed-rate debt1,286,9661,559,034
Variable- rate debt1,133,107764,647
Debt, net2,420,0732,323,681
Total Estimated Fair Value
  
Carrying amount and estimated fair value of debt  
Fixed-rate debt1,290,5061,579,022
Variable- rate debt1,132,081769,247
Debt, net$ 2,422,587$ 2,348,269