Debt (Schedule Of Credit Facilities) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Line of Credit Facility [Line Items]
Line of credit facilities, maximum amount outstanding during period	$ 303,100	$ 330,700
Line of credit facilities, weighted average balance	157,447	151,814
Year-to-date weighted average interest rate (excluding facility fee)	1.30%	1.50%
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	30,000	145,000
Available balance	467,571	351,571
Letter of credit outstanding under facility	2,429	3,429
Variable interest rate (excluding facility fee)	1.10%	1.30%
Unsecured And Uncommitted Overnight Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 36,000	$ 21,500
Variable interest rate (excluding facility fee)	1.50%	1.50%