Organization of The Parent Company and The Operating Partnership (Details)	Sep. 30, 2012 A
Organization of The Parent Company and The Operating Partnership (Textuals) [Abstract]
Ownership in the Operating Partnership	98.20%
Area of undeveloped parcels of land (in acres) owned by a Real Estate Venture	24
Parent Company [Member]
Organization of The Parent Company and The Operating Partnership (Textuals) [Abstract]
Net rentable square feet	24,500,000
Area of undeveloped parcels of land (in acres) owned by a Real Estate Venture	424
Area of additional undeveloped parcels of land with option to purchase	52
Unconsolidated real estate ventures [Member]
Organization of The Parent Company and The Operating Partnership (Textuals) [Abstract]
Number of unconsolidated real estate ventures	19
Net rentable square feet	7,000,000
Wholly-owned management company subsidiaries [Member]
Organization of The Parent Company and The Operating Partnership (Textuals) [Abstract]
Net rentable square feet	31,900,000
Third Parties and Unconsolidated Real Estate Ventures [Member]
Organization of The Parent Company and The Operating Partnership (Textuals) [Abstract]
Net rentable square feet	7,400,000
Total Properties [Member]
Organization of The Parent Company and The Operating Partnership (Textuals) [Abstract]
Number of properties with ownership interest	219
Office Properties [Member]
Organization of The Parent Company and The Operating Partnership (Textuals) [Abstract]
Number of properties with ownership interest	194
Industrial facilities [Member]
Organization of The Parent Company and The Operating Partnership (Textuals) [Abstract]
Number of properties with ownership interest	19
Mixed-use properties [Member]
Organization of The Parent Company and The Operating Partnership (Textuals) [Abstract]
Number of properties with ownership interest	5
Development Property [Member]
Organization of The Parent Company and The Operating Partnership (Textuals) [Abstract]
Number of properties with ownership interest	1