Intangible Assets on Real Estate Acquisitions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 222,683	$ 223,054
Accumulated Amortization	163,591	144,703
Net Carrying Amount	59,092	78,351
Amortization of the intangible assets	$ 19,300	20,000	$ 18,500
Weighted average amortization period of intangible assets	11 years
Estimated future amortization expense associated with the intangible asset categories for the next five years
2018	$ 14,500
2019	8,100
2020	5,700
2021	5,500
2022	4,100
In-place lease value [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	132,276	132,647
Accumulated Amortization	110,814	99,940
Net Carrying Amount	$ 21,462	32,707
Weighted average amortization period of intangible assets	6 years
Tenant relationship value [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 60,028	60,028
Accumulated Amortization	32,198	26,253
Net Carrying Amount	$ 27,830	33,775
Weighted average amortization period of intangible assets	8 years
Below-market cost arrangements [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 15,102	15,102
Accumulated Amortization	7,507	7,285
Net Carrying Amount	$ 7,595	7,817
Weighted average amortization period of intangible assets	34 years
Above-market leases [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 13,944	13,944
Accumulated Amortization	12,092	10,259
Net Carrying Amount	$ 1,852	3,685
Weighted average amortization period of intangible assets	3 years
Other [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 1,333	1,333
Accumulated Amortization	980	966
Net Carrying Amount	$ 353	$ 367
Weighted average amortization period of intangible assets	25 years