Notes Payable and Unsecured Credit Facility (Schedule of Debt) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Notes payable:
Notes payable, principal amount outstanding	$ 451,169	$ 486,812
Unamortized deferred financing costs related to notes payable	(1,675)	(2,542)
Notes payable, net of deferred financing costs	449,494	484,270
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	433,000	358,000
Unamortized deferred financing costs related to the term loan of the unsecured credit facility	(1,101)	(1,789)
Unsecured credit facility, net of deferred financing costs	431,899	356,211
Total debt outstanding	881,393	840,481
Revolving Line of Credit [Member]
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	223,000	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	101,918	114,783
Variable Rate Debt, Subject To Interest Rate Swap [Member]
Notes payable:
Notes payable, principal amount outstanding	308,710	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	40,541	41,604
Unsecured credit facility:
Unsecured credit facility, principal amount outstanding	$ 223,000	$ 148,000