Debt (Schedule Of Credit Facilities) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Balance outstanding	$ 200,600	$ 245,000
Maximum balance outstanding during the period	245,000	372,000
Weighted average balance	$ 189,875	$ 141,017
Year-to-date weighted average interest rate (excluding facility fee)	1.70%	1.30%
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 200,000	$ 245,000
Available balance	$ 294,386	$ 250,140
Variable interest rate (excluding facility fee)	1.90%	1.50%
Letters of Credit [Member]
Line of Credit Facility [Line Items]
Letters of credit outstanding under facility	$ 5,614	$ 4,860
Unsecured And Uncommitted Overnight Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 560	$ 0
Variable interest rate (excluding facility fee)	2.50%	0.00%