Acquired Intangible Assets and Liabilities (Summary of Acquired Intangible Assets) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Below-market lease, gross amount	$ 445,551	$ 469,119
Below-market lease, accumulated amortization	(229,031)	(219,654)
Total	216,520	249,465
Acquired in-place lease value
Finite-Lived Intangible Assets [Line Items]
Gross amount	1,464,461	1,473,515
Accumulated amortization	(709,645)	(613,948)
Net	754,816	859,567
Tenant relationship value
Finite-Lived Intangible Assets [Line Items]
Gross amount	1,971,915	1,978,277
Accumulated amortization	(231,111)	(169,919)
Net	1,740,804	1,808,358
Acquired above-market leases
Finite-Lived Intangible Assets [Line Items]
Gross amount	281,263	294,514
Accumulated amortization	(131,179)	(110,139)
Net	$ 150,084	$ 184,375