Debt (Schedule of Credit Facilities) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Balance outstanding	$ 73,000	$ 189,000
Maximum balance outstanding during the period	244,500	270,000
Weighted average balance	$ 118,220	$ 151,036
Year-to-date weighted average interest rate (excluding facility fee)	0.90%	0.80%
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 63,000	$ 189,000
Available balance	$ 432,656	$ 306,777
Variable interest rate (excluding facility fee)	0.80%	0.80%
Letters of Credit [Member]
Line of Credit Facility [Line Items]
Letters of credit outstanding under facility	$ 4,344	$ 4,223
Short-Term Unsecured Facility [Member]
Line of Credit Facility [Line Items]
Balance outstanding	$ 10,000
Variable interest rate (excluding facility fee)	1.40%