Investment Property (Intangible Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Finite Lived Intangible Assets [Line Items]
Net	$ 393,895	$ 406,257
Out-of-Market Lease Liabilities
Out-of-Market Lease Liabilities, Cost	(107,984)	(108,043)
Out-of-Market Lease Liabilities, Less: accumulated amortization	42,372	38,477
Out-of-Market Lease Liabilities, Net	(65,612)	(69,566)
In-Place Leases
Finite Lived Intangible Assets [Line Items]
Cost	652,711	662,854
Less: accumulated amortization	(284,353)	(283,774)
Net	368,358	379,080
Out-of-Market Lease Assets
Finite Lived Intangible Assets [Line Items]
Cost	66,116	69,510
Less: accumulated amortization	(40,579)	(42,333)
Net	$ 25,537	$ 27,177