Notes Payable and Unsecured Credit Facility (Schedule of Debt) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Notes payable:
Notes payable, principal amount outstanding	$ 478,352	$ 486,812
Unamortized deferred financing costs related to notes payable	(2,160)	(2,542)
Notes payable, net of deferred financing costs	476,192	484,270
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	378,000	358,000
Unamortized deferred financing costs related to the term loan of the unsecured credit facility	(1,506)	(1,789)
Unsecured credit facility, net of deferred financing costs	376,494	356,211
Total debt outstanding	852,686	840,481
Revolving Line of Credit [Member]
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	168,000	148,000
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	108,472	114,783
Variable Rate Debt, Subject To Interest Rate Swap [Member]
Notes payable:
Notes payable, principal amount outstanding	328,643	330,425
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	210,000	210,000
Variable Rate [Member]
Notes payable:
Notes payable, principal amount outstanding	41,237	41,604
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	$ 168,000	$ 148,000