Annual report pursuant to Section 13 and 15(d)

Unsecured Lines Of Credit Payable - Narrative (Details)

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Unsecured Lines Of Credit Payable - Narrative (Details) (USD $)
12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2012
Credit Facility No. 1 [Member]
Dec. 31, 2012
Credit Facility No. 1 [Member]
London Interbank Offered Rate (LIBOR) [Member]
Dec. 31, 2012
Credit Facility No. 1 [Member]
Maximum [Member]
Dec. 31, 2012
Credit Facility No. 2 [Member]
Dec. 31, 2012
Credit Facility No. 2 [Member]
London Interbank Offered Rate (LIBOR) [Member]
Dec. 31, 2012
Credit Facility No. 2 [Member]
Maximum [Member]
Dec. 31, 2012
Unsecured Debt [Member]
10 Year Unsecured Notes 5.05 % [Member]
Apr. 30, 2012
Unsecured Debt [Member]
10 Year Unsecured Notes 5.05 % [Member]
Dec. 31, 2012
Unsecured Debt [Member]
10 Year Unsecured Notes 3.950 % [Member]
Sep. 30, 2010
Unsecured Debt [Member]
10 Year Unsecured Notes 3.950 % [Member]
Line of Credit Facility [Line Items]                          
Committed capacity $ 500,000,000 $ 475,000,000 $ 337,000,000 $ 100,000,000   $ 200,000,000 $ 400,000,000   $ 600,000,000        
Stated interest rate   3.875%               5.05% 5.05% 3.95% 3.95%
Expiration date       Jun. 30, 2015     Jul. 31, 2016            
Spread on variable rate         1.20%     1.20%          
Extension term 1 year                        
Facility fee 0.25%