Investments in Unconsolidated Real Estate Funds - Narrative (Details) - 6 months ended Jun. 30, 2018 ft² in Millions	ft² fund	Total	office_property	property
Schedule of Equity Method Investments [Line Items]
Number of properties				81
Percentage of amounts related to the Fund		100.00%
Opportunity Fund
Schedule of Equity Method Investments [Line Items]
Equity interest of the Fund, percent		6.20%
Fund X
Schedule of Equity Method Investments [Line Items]
Equity interest of the Fund, percent		69.40%
Partnership X
Schedule of Equity Method Investments [Line Items]
Equity interest of the Fund, percent		24.30%
Unconsolidated Fund properties
Schedule of Equity Method Investments [Line Items]
Number of real estate funds owned and managed | fund	3
Office Building
Schedule of Equity Method Investments [Line Items]
Number of properties				71
Office Building | Unconsolidated Fund properties
Schedule of Equity Method Investments [Line Items]
Number of properties			8	8
Area of real estate portfolio (sq ft) | ft²	1.8