Acquired Intangible Assets and Liabilities (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Amortization of acquired leases	$ (27,300)	$ (2,200)	$ 8,300
Expected average remaining lives of acquired below market leases	8 years 21 days
Amortization of intangible assets	$ 416,621	$ 247,468	180,608
Acquired above-market leases
Finite-Lived Intangible Assets [Line Items]
Expected average remaining lives of acquired above market leases	2 years 10 months 1 day	65 months
Acquired in-place lease value
Finite-Lived Intangible Assets [Line Items]
Expected average remaining lives of acquired above market leases	6 years 4 months 10 days	81 months
Amortization of intangible assets	$ 211,000	$ 101,200	52,600
Weighted average remaining contractual life for acquired leases excluding renewals or extensions	5 years 11 months 10 days
Tenant Relationship Value
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 123,500	85,900	67,200
Weighted average remaining contractual life for acquired leases excluding renewals or extensions	14 years 10 months 4 days
Trade Name
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 0	$ 0	6,900
Write-down of intangible assets			$ 6,100