Real Estate and In-Place Lease Value - Gains on Sale of Real Estate, Net (Details) $ in Millions	6 Months Ended
Jun. 30, 2019 USD ($) property	Jun. 30, 2018 USD ($)
Real Estate [Line Items]
Gain on sale of real estate, net	$ 246.0	$ 267.2
Gain on sale of real estate, attributable to noncontrolling interest	117.0
Real Estate, Excluding Development Project [Member]
Real Estate [Line Items]
Gain on sale of real estate, net	$ 246.7
Commercial [Member] \| United Kingdom [Member]
Real Estate [Line Items]
Number of real estate properties sold \| property	6
Multifamily [Member] \| Western U.S. [Member]
Real Estate [Line Items]
Number of real estate properties sold \| property	5

The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109226691&loc=d3e2941-110230

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2443-110228

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration