Debt (Schedule Of Credit Facilities) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Balance outstanding	$ 180,000	$ 149,500
Maximum balance outstanding during the period	200,000	244,500
Weighted average balance	$ 147,446	$ 100,506
Year-to-date weighted average interest rate (excluding facility fee)	1.30%	0.90%
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 180,000	$ 140,000
Available balance	$ 315,190	$ 355,190
Variable interest rate (excluding facility fee)	1.30%	1.30%
Letters of Credit [Member]
Line of Credit Facility [Line Items]
Letters of credit outstanding under facility	$ 4,810	$ 4,810
Unsecured And Uncommitted Overnight Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 0	$ 9,500
Variable interest rate (excluding facility fee)	0.00%	1.70%