Intangible Assets - Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Intangible Assets [Line Items]
Intangible Assets, Total Cost	$ 123,806	$ 160,681
Intangible Assets, Accumulated Amortization	(58,834)	(88,203)
Intangible Assets, net	64,972	72,478
Acquired Lease Intangibles, Net	20,274	18,119
Below market leases [Member]
Intangible Assets [Line Items]
Acquired Lease Intangibles, Gross	36,213	37,579
Acquired Lease Intangibles, Accumulated Amortization	(15,939)	(19,460)
Acquired Lease Intangibles, Net	20,274	18,119
In-place lease value [Member]
Intangible Assets [Line Items]
Intangible Assets, Total Cost	108,060	142,889
Intangible Assets, Accumulated Amortization	(47,003)	(75,696)
Intangible Assets, net	61,057	67,193
Tenant relationship value [Member]
Intangible Assets [Line Items]
Intangible Assets, Total Cost	11,201	13,074
Intangible Assets, Accumulated Amortization	(9,275)	(10,167)
Intangible Assets, net	1,926	2,907
Above market leases acquired [Member]
Intangible Assets [Line Items]
Intangible Assets, Total Cost	4,545	4,718
Intangible Assets, Accumulated Amortization	(2,556)	(2,340)
Intangible Assets, net	$ 1,989	$ 2,378