Notes Payable and Unsecured Credit Facility (Schedule of Debt Outstanding) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Notes payable:
Notes payable, principal amount outstanding	$ 505,979	$ 543,332
Unamortized deferred financing costs related to notes payable	(3,334)	(4,261)
Notes payable, net of deferred financing costs	502,645	539,071
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	428,000	293,000
Unamortized deferred financing costs related to unsecured credit facility	(2,190)	(2,544)
Unsecured credit facility, net of deferred financing costs	425,810	290,456
Total debt outstanding	928,455	829,527
Revolving Line of Credit [Member]
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	218,000	83,000
Term Loan [Member]
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	210,000	210,000
Fixed Rate [Member]
Notes payable:
Notes payable, principal amount outstanding	129,665	131,233
Variable Rate Debt, Subject To Interest Rate Swap [Member]
Notes payable:
Notes payable, principal amount outstanding	333,966	369,002
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	210,000	55,000
Variable Rate Debt, Subject To Interest Rate Swap [Member] | Term Loan [Member]
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	210,000
Variable Rate [Member]
Notes payable:
Notes payable, principal amount outstanding	42,348	43,097
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	$ 218,000	$ 238,000