Acquired Intangible Assets and Liabilities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Amortization of below market leases	$ (6,800)	$ (700)	$ (20,600)	$ (4,700)
Expected average remaining lives of acquired below market leases (in years)			8 years 3 months 12 days
Amortization of acquired in-place lease value and deferred leasing costs			$ 317,355	145,262
Acquired in-place lease value
Finite-Lived Intangible Assets [Line Items]
Expected average remaining lives (in years)			6 years 2 months 2 days
Amortization of acquired in-place lease value and deferred leasing costs	50,200	20,100	$ 162,300	48,700
Weighted average remaining contractual life for acquired leases excluding renewals or extensions (in years)			5 years 8 months 4 days
Tenant relationship value
Finite-Lived Intangible Assets [Line Items]
Amortization of acquired in-place lease value and deferred leasing costs	$ 30,800	$ 19,900	$ 92,800	$ 55,000
Weighted average remaining contractual life for acquired leases excluding renewals or extensions (in years)			14 years 6 months 1 day
Acquired above-market leases
Finite-Lived Intangible Assets [Line Items]
Expected average remaining lives (in years)			3 years 11 days