Acquired Intangible Assets and Liabilities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Amortization of below market leases		$ (2,200)	$ (8,300)	$ (9,300)
Amortization of intangible assets		$ 247,468	180,608	105,833
Expected average remaining lives of acquired below market leases		6 years 7 months 10 days
Acquired in-place lease value
Finite-Lived Intangible Assets [Line Items]
Expected average remaining lives		9 years 2 months 13 days
Amortization of intangible assets		$ 101,200	52,600	46,200
Weighted average remaining contractual life for acquired leases excluding renewals or extensions		6 years 27 days
Acquired above-market leases:
Finite-Lived Intangible Assets [Line Items]
Expected average remaining lives		3 years 11 months 5 days
Tenant relationship value
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets		$ 85,900	67,200	14,500
Remaining amortization period		15 years 3 months 3 days
Trade name:
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets		$ 0	$ 6,900	$ 400
Write-down of intangible assets	$ 6,100