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Fair Value Disclosure of Financial Instruments - Additional Information (Detail) (USD $)
Jun. 30, 2012
Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Notes Payable $ 1,589,421,000 $ 1,649,755,000
Fixed Rate Debt
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Notes Payable 569,000,000 538,000,000
Notes Payable, fair value 613,000,000 560,000,000
Variable Rate Debt
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Notes Payable 1,030,000,000 1,112,000,000
Notes Payable, fair value $ 983,000,000 $ 1,053,000,000