Acquired Intangible Assets and Liabilities (Summary of Acquired Intangible Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Acquired below-market leases:
Gross amount	$ 469,119	$ 283,899
Accumulated amortization	(219,654)	(202,000)
Net	249,465	81,899
Acquired in-place lease value
Finite-Lived Intangible Assets [Line Items]
Gross amount	1,473,515	896,693
Accumulated amortization	(613,948)	(517,443)
Net	859,567	379,250
Tenant relationship value
Finite-Lived Intangible Assets [Line Items]
Gross amount	1,978,277	971,519
Accumulated amortization	(169,919)	(82,069)
Net	1,808,358	889,450
Trade name:
Finite-Lived Intangible Assets [Line Items]
Gross amount	0	7,300
Accumulated amortization	0	(7,300)
Net	0	0
Acquired above-market leases:
Finite-Lived Intangible Assets [Line Items]
Gross amount	294,514	110,142
Accumulated amortization	(110,139)	(87,961)
Net	$ 184,375	$ 22,181