Debt (Schedule Of Credit Facilities) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Line of Credit Facility [Line Items]
Maximum balance outstanding during the period	$ 265,500	$ 303,100
Weighted average balance	114,956	157,447
Year-to-date weighted average interest rate (excluding facility fee)	1.10%	1.30%
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	0	30,000
Available balance	497,571	467,571
Letter of credit outstanding under facility	2,429	2,429
Variable interest rate (excluding facility fee)	0.00%	1.10%
Unsecured And Uncommitted Overnight Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 0	$ 36,000
Variable interest rate (excluding facility fee)	0.00%	1.50%