Acquired Intangible Assets and Liabilities (Summary of Acquired Intangible Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Below-market lease, gross amount	$ 443,457	$ 469,119
Below-market lease, accumulated amortization	(235,255)	(219,654)
Total	208,202	249,465
Acquired in-place lease value
Finite-Lived Intangible Assets [Line Items]
Gross amount	1,452,050	1,473,515
Accumulated amortization	(748,087)	(613,948)
Net	703,963	859,567
Tenant relationship value
Finite-Lived Intangible Assets [Line Items]
Gross amount	1,969,111	1,978,277
Accumulated amortization	(261,618)	(169,919)
Net	1,707,493	1,808,358
Acquired above-market leases
Finite-Lived Intangible Assets [Line Items]
Gross amount	280,399	294,514
Accumulated amortization	(145,272)	(110,139)
Net	$ 135,127	$ 184,375