Intangible Assets and Liabilities - Summary of Intangible Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Assets
Gross	$ 780,677	$ 534,108
Accumulated amortization	(299,398)	(256,305)
Total	$ 481,279	$ 277,803
Weighted Average Remaining Amortization in Years	19 years 2 months 12 days	16 years 1 month 6 days
Liabilities
Gross	$ 82,398	$ 46,002
Accumulated amortization	(12,546)	(8,946)
Total	$ 69,852	$ 37,056
Weighted Average Remaining Amortization in Years	24 years 10 months 24 days	28 years 6 months
Below market leases
Liabilities
Gross	$ 61,788	$ 34,370
Weighted Average Remaining Amortization in Years	14 years 8 months 12 days	18 years 7 months 6 days
Above market leasehold interests
Liabilities
Gross	$ 20,610	$ 11,632
Weighted Average Remaining Amortization in Years	50 years 3 months 18 days	53 years
In place leases
Assets
Gross	$ 478,052	$ 294,597
Weighted Average Remaining Amortization in Years	9 years 8 months 12 days	9 years 8 months 12 days
Tenant relationships
Assets
Gross	$ 170,539	$ 172,974
Weighted Average Remaining Amortization in Years	10 years 8 months 12 days	10 years 7 months 6 days
Above market leases
Assets
Gross	$ 39,724	$ 28,401
Weighted Average Remaining Amortization in Years	6 years 3 months 18 days	6 years 3 months 18 days
Below market leasehold interests
Assets
Gross	$ 92,362	$ 38,136
Weighted Average Remaining Amortization in Years	63 years 4 months 24 days	60 years 4 months 24 days