Derivative Financial Instruments - Floating-to-Fixed Interest Rate Swaps (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Amount	$ 647,326	$ 513,208
Interest rate swaps
Derivative [Line Items]
Notional Amount	247,326
Not designated as accounting hedges
Derivative [Line Items]
Notional Amount	500,000	450,000
Not designated as accounting hedges | Interest rate swaps
Derivative [Line Items]
Notional Amount	100,000	100,000
Not designated as accounting hedges | Senior Unsecured Term Loan 2.00% | Interest rate swaps
Derivative [Line Items]
Notional Amount	$ 50,000
Fixed interest rate	2.00%
Effective interest rate	3.50%
Not designated as accounting hedges | Senior Unsecured Term Loan 1.50% | Interest rate swaps
Derivative [Line Items]
Notional Amount	$ 50,000
Fixed interest rate	2.78%
Effective interest rate	4.28%
Designated as accounting hedge | Interest rate swaps
Derivative [Line Items]
Notional Amount	$ 147,326	$ 63,208
Designated as accounting hedge | John Hopkins Village | Interest rate swaps
Derivative [Line Items]
Notional Amount	$ 52,256
Fixed interest rate	2.94%
Effective interest rate	4.19%
Designated as accounting hedge | Lightfoot Marketplace | Interest rate swaps
Derivative [Line Items]
Notional Amount	$ 10,500
Fixed interest rate	3.02%
Effective interest rate	4.77%
Designated as accounting hedge | 249 Central Park Retail, South Retail, and Fountain Plaza Retail | Interest rate swaps
Derivative [Line Items]
Notional Amount	$ 34,570
Fixed interest rate	2.25%
Effective interest rate	3.85%
Designated as accounting hedge | Senior Unsecured Term Loan 2.26% | Interest rate swaps
Derivative [Line Items]
Notional Amount	$ 50,000
Fixed interest rate	2.26%
Effective interest rate	3.76%