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Debt (Schedule Of Credit Facilities) (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Line of Credit Facility [Line Items]    
Balance outstanding $ 125,000 $ 245,000
Maximum balance outstanding during the period 245,000 372,000
Weighted average balance $ 169,263 $ 141,017
Year-to-date weighted average interest rate (excluding facility fee) 1.80% 1.30%
Unsecured Revolving Credit Facility [Member]    
Line of Credit Facility [Line Items]    
Balance outstanding $ 125,000 $ 245,000
Available balance $ 368,610 $ 250,140
Variable interest rate (excluding facility fee) 2.00% 1.50%
Letters of Credit [Member]    
Line of Credit Facility [Line Items]    
Letters of credit outstanding under facility $ 6,390 $ 4,860
Unsecured And Uncommitted Overnight Facility [Member]    
Line of Credit Facility [Line Items]    
Balance outstanding $ 0 $ 0
Variable interest rate (excluding facility fee) 0.00% 0.00%