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Debt (Schedule Of Credit Facilities) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]    
Maximum balance outstanding during the period $ 0 $ 245,000
Weighted average balance $ 0 $ 133,386
Year-to-date weighted average interest rate (excluding facility fee) 0.00% 1.80%
Unsecured Revolving Credit Facility [Member]    
Line of Credit Facility [Line Items]    
Balance outstanding $ 0 $ 0
Available balance $ 497,946 $ 493,610
Variable interest rate (excluding facility fee) 0.00% 0.00%
Letters of Credit [Member]    
Line of Credit Facility [Line Items]    
Letters of credit outstanding under facility $ 2,054 $ 6,390
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%