Intangible Assets and Liabilities - Summary of Intangible Assets and Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Assets
Gross	$ 765,487	$ 771,643
Accumulated amortization	(344,214)	(312,655)
Total	$ 421,273	$ 458,988
Weighted Average Remaining Amortization in Years	20 years 10 months 18 days	19 years 6 months
Liabilities
Gross	$ 82,562	$ 82,430
Accumulated amortization	(17,598)	(14,227)
Total	$ 64,964	$ 68,203
Weighted Average Remaining Amortization in Years	24 years 10 months 18 days	25 years
Below market leases
Liabilities
Gross	$ 61,952	$ 61,820
Weighted Average Remaining Amortization in Years	14 years 6 months	14 years 8 months 12 days
Above market leasehold interests
Liabilities
Gross	$ 20,610	$ 20,610
Weighted Average Remaining Amortization in Years	49 years 8 months 12 days	50 years 1 month 6 days
In place leases
Assets
Gross	$ 471,268	$ 474,252
Weighted Average Remaining Amortization in Years	9 years 8 months 12 days	9 years 9 months 18 days
Tenant relationships
Assets
Gross	$ 162,670	$ 164,947
Weighted Average Remaining Amortization in Years	10 years 3 months 18 days	10 years 2 months 12 days
Above market leases
Assets
Gross	$ 39,187	$ 40,082
Weighted Average Remaining Amortization in Years	6 years 2 months 12 days	6 years 3 months 18 days
Below market leasehold interests
Assets
Gross	$ 92,362	$ 92,362
Weighted Average Remaining Amortization in Years	64 years 7 months 18 days	63 years 4 months 24 days