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Noncontrolling Interests (Property Partnerships) (Narrative) (Details) - USD ($) $ in Thousands | May 12, 2016 | Sep. 30, 2017 | Jun. 06, 2017 | Dec. 31, 2016
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Noncontrolling Interest [Line Items] |  |  |  |
Noncontrolling Interest in Limited Partnerships |  | $ 1,668,675 |  | $ 1,530,647
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 |  | $ 15,200 |  |
767 Fifth Avenue (the General Motors Building) | Consolidated Properties [Member] |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |
Noncontrolling Interest in Limited Partnerships |  |  | $ 109,600 |
Ownership Percentage by the Company |  |  | 60.00% |
Unpaid principal balance of the members' notes payable contributed to equity |  |  | $ 273,900 |
767 Fifth Avenue (the General Motors Building) | Consolidation, Eliminations [Member] | Consolidated Properties [Member] |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |
Unpaid principal balance of the members' notes payable contributed to equity |  |  | $ 164,400 |

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