Acquired Intangible Assets and Liabilities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Increase (decrease) in rental revenues	$ (6,200)	$ (6,900)
Expected average remaining lives of acquired below market leases (in years)	7 years 11 months 16 days
Amortization of acquired in-place lease value and deferred leasing costs	$ 103,934	108,358
Acquired above-market leases
Finite-Lived Intangible Assets [Line Items]
Expected average remaining lives (in years)	2 years 8 months 1 day
Acquired in-place lease value
Finite-Lived Intangible Assets [Line Items]
Expected average remaining lives (in years)	5 years 11 months 1 day
Amortization of acquired in-place lease value and deferred leasing costs	$ 44,900	57,000
Weighted average remaining contractual life for acquired leases excluding renewals or extensions (in years)	5 years 5 months 15 days
Tenant relationship value
Finite-Lived Intangible Assets [Line Items]
Amortization of acquired in-place lease value and deferred leasing costs	$ 38,000	$ 31,000
Weighted average remaining contractual life for acquired leases excluding renewals or extensions (in years)	14 years 1 day