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Unsecured Debt (Details 1) (USD $)
In Thousands, unless otherwise specified
 6 Months Ended 12 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Summary of short-term bank borrowings under unsecured commercial bank credit facility    
Total revolving credit facility $ 932,800  
Borrowings outstanding at end of period 800,000  
Unsecured Commercial Bank Credit Facility [Member]
    
Summary of short-term bank borrowings under unsecured commercial bank credit facility    
Total revolving credit facility 900,000 900,000
Borrowings outstanding at end of period 0 [1] 421,000 [1]
Weighted average daily borrowings during the period ended 319,923 227,498
Maximum daily borrowings during the period ended $ 788,000 $ 450,000
Weighted average interest rate during the period ended 1.50% 1.00%
Interest rate at the end of the period   1.50%
[1] Excludes $4.1 million of letters of credit at June 30, 2012.