Debt (Schedule Of Credit Facilities) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Balance outstanding	$ 130,000	$ 149,500
Maximum balance outstanding during the period	372,000	244,500
Weighted average balance	$ 133,738	$ 100,506
Year-to-date weighted average interest rate (excluding facility fee)	1.40%	0.90%
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 128,000	$ 140,000
Available balance	$ 367,140	$ 355,190
Variable interest rate (excluding facility fee)	1.40%	1.30%
Letters of Credit [Member]
Line of Credit Facility [Line Items]
Letters of credit outstanding under facility	$ 4,860	$ 4,810
Unsecured And Uncommitted Overnight Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 2,000	$ 9,500
Variable interest rate (excluding facility fee)	1.80%	1.70%