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Noncontrolling Interests (Property Partnerships) (Narrative) (Details) - USD ($) $ in Thousands | May 12, 2016 | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Noncontrolling Interest [Line Items] |  |  |
Noncontrolling Interest in Limited Partnerships |  | $ 1,685,715 | $ 1,683,760
Salesforce Tower[Member] | Consolidated Properties [Member] |  |  |
Noncontrolling Interest [Line Items] |  |  |
Portion of project costs covered by a construction loan | 50.00% |  |
Portion of costs funded (in percentage) | 50.00% |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 5.00% |  |
Debt Instrument, Basis Spread on Variable Rate | 3.00% |  |
Salesforce Tower[Member] | Parent Company [Member] | Consolidated Properties [Member] |  |  |
Noncontrolling Interest [Line Items] |  |  |
Costs funded (in percentage) | 2.50% |  |
Preferred equity funded |  | $ 17,200 |

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