Notes Payable and Unsecured Credit Facility (Schedule of Debt) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Notes payable:
Notes payable	$ 549,825	$ 490,909
Unsecured credit facility:
Credit facility	208,000	75,000
Total debt outstanding	757,825	565,909
Fixed Rate [Member]
Notes payable:
Notes payable	132,762	134,271
Variable Rate, Subject To Interest Rate Swap [Member]
Notes payable:
Notes payable	240,046	206,162
Unsecured credit facility:
Credit facility	55,000	55,000
Variable Rate [Member]
Notes payable:
Notes payable	177,017	150,476
Unsecured credit facility:
Credit facility	153,000	20,000
Revolving Line Of Credit [Member]
Unsecured credit facility:
Credit facility	133,000	0
Term Loan [Member]
Unsecured credit facility:
Credit facility	$ 75,000	$ 75,000