Organization of the Parent Company and The Operating Partnership (Details) ft² in Millions	Sep. 30, 2015 a ft² property real_estate_investment	Sep. 30, 2014 ft²
Organization of The Parent Company and The Operating Partnership [Line Items]
Area owned by real estate venture of undeveloped parcels of land | a	4.4
Area owned by real estate venture in parcels of land under development | a	22.5
Parent Company [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Ownership in the Operating Partnership	99.10%
Net Rentable Square Feet | ft²	24.8
Area Owned by Company of Undeveloped Parcels Of Land | a	413
Area Of Additional Undeveloped Parcels Of Land With Option to Purchase | a	63
Total Potential Development Capacity | ft²	7.8
Unconsolidated Real Estate Ventures [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Net Rentable Square Feet | ft²	4.0	5.9
Number of Unconsolidated Real Estate Ventures | real_estate_investment	17
Wholly-owned Management Company Subsidiaries [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Net Rentable Square Feet | ft²	30.9
Third Parties and Real Estate Ventures [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Net Rentable Square Feet | ft²	6.2
Total Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties	192
Office Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties	155
Industrial Facilities [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties	20
Mixed Use Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties	4
Retail Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties	1
Core Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties	180
Assets Held-for-sale [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties	6
Development Property [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties	4
Redevelopment Property [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties	1
Re-entitlement Property [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties	1
Wholly Owned Properties [Member] | Parent Company [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Net Rentable Square Feet | ft²	24.7