Mortgage Notes Payable (Summary of Mortgage Notes Payable) (Details) - Mortgage notes payable and premiums, net $ in Thousands	Jun. 30, 2016 USD ($) property	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Encumbered Properties | property	451
Outstanding Loan Amount	$ 1,039,759	$ 1,053,648
Effective Interest Rate	4.76%	4.77%
Deferred financing costs, net of accumulated amortization	$ (17,728)	$ (20,066)
Mortgage notes payable, net of deferred financing costs	$ 1,022,031	1,033,582
SAAB Sensis I
Debt Instrument [Line Items]
Encumbered Properties | property	1
Outstanding Loan Amount	$ 8,018	$ 8,190
Effective Interest Rate	6.01%	6.01%
SunTrust Bank II
Debt Instrument [Line Items]
Encumbered Properties | property	30
Outstanding Loan Amount	$ 25,000	$ 25,000
Effective Interest Rate	5.50%	5.50%
C&S Wholesale Grocer I
Debt Instrument [Line Items]
Encumbered Properties | property	4
Outstanding Loan Amount	$ 82,313	$ 82,313
Effective Interest Rate	5.56%	5.56%
SunTrust Bank III
Debt Instrument [Line Items]
Encumbered Properties | property	116
Outstanding Loan Amount	$ 90,045	$ 99,677
Effective Interest Rate	5.50%	5.50%
SunTrust Bank IV
Debt Instrument [Line Items]
Encumbered Properties | property	27
Outstanding Loan Amount	$ 21,243	$ 25,000
Effective Interest Rate	5.50%	5.50%
Stop & Shop I
Debt Instrument [Line Items]
Encumbered Properties | property	4
Outstanding Loan Amount	$ 38,608	$ 38,936
Effective Interest Rate	5.63%	5.63%
Sanofi US I - New Loan | Sanofi US I
Debt Instrument [Line Items]
Encumbered Properties | property	1
Outstanding Loan Amount	$ 125,000	$ 125,000
Effective Interest Rate	5.16%	5.16%
Multi-Tenant Mortgage Loan
Debt Instrument [Line Items]
Encumbered Properties | property	268
Outstanding Loan Amount	$ 649,532	$ 649,532
Effective Interest Rate	4.36%	4.36%