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Debt Structure (Detail) (USD $)
3 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2012
Debt Instrument [Line Items]      
Unsecured notes payable $ 577,000,000 $ 483,000,000  
Total Outstanding Debt 1,671,102,000   1,653,646,000
Effective Rate 3.60%    
Contract Maturity Apr. 19, 2018    
Tax Free Credit Facility [Member]
     
Debt Instrument [Line Items]      
Total Outstanding Debt 20,000,000    
Unsecured Debt [Member]
     
Debt Instrument [Line Items]      
Effective Rate 3.40%    
Contract Maturity Apr. 06, 2019    
Secured Debt [Member]
     
Debt Instrument [Line Items]      
Total Outstanding Debt 1,094,102,000    
Unsecured Debt [Member]
     
Debt Instrument [Line Items]      
Total Outstanding Debt 577,000,000    
Fixed Rate Debt | Unsecured Debt [Member]
     
Debt Instrument [Line Items]      
Effective Rate 2.40%    
Contract Maturity Mar. 01, 2017    
Fixed Rate Debt | Term Fixed Loan With Swaps [Member] | Unsecured Debt [Member]
     
Debt Instrument [Line Items]      
Total Outstanding Debt 150,000,000    
Fixed Rate Debt | Secured Debt [Member]
     
Debt Instrument [Line Items]      
Total Outstanding Debt 755,116,000    
Variable Rate Debt | Unsecured Debt [Member]
     
Debt Instrument [Line Items]      
Total Outstanding Debt 117,000,000    
Effective Rate 1.50%    
Contract Maturity Nov. 01, 2015    
Variable Rate Debt | Secured Debt [Member]
     
Debt Instrument [Line Items]      
Total Outstanding Debt $ 338,986,000