Fair Value of Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Non-Recurring Fair Value Measurements:
Long-lived assets held and used	$ 6,310,062	$ 5,506,997
Office
Non-Recurring Fair Value Measurements:
Long-lived assets held and used	4,989,157	4,216,114
Number of long-lived held and used properties impaired	6
Industrial and Other
Non-Recurring Fair Value Measurements:
Long-lived assets held and used	1,320,905	1,290,883
Industrial
Non-Recurring Fair Value Measurements:
Number of long-lived held and used properties impaired	20
Interest Rate Swap
Recurring Fair Value Measurements:
Effective portion of interest rate contracts	(15,796)	(6,956)
Non-Recurring Fair Value Measurements:
Interest rate swap agreements qualifying as cash flow hedges used for hedging mortgage note payable	175,000
Derivative, variable rate basis	LIBOR
Decrease in fair value of interest rate cash flow hedges	8,840	9,501
Carrying Amount
Non-Recurring Fair Value Measurements:
Long-lived assets held and used	42,322
Recurring | Total Fair value
Recurring Fair Value Measurements:
Effective portion of interest rate contracts	(15,796)
Recurring | Fair Value at Reporting Date Using Significant Other Observable Inputs (Level 2)
Recurring Fair Value Measurements:
Effective portion of interest rate contracts	(15,796)
Nonrecurring
Non-Recurring Fair Value Measurements:
Number of long-lived held and used properties impaired	21
Nonrecurring | Other markets
Non-Recurring Fair Value Measurements:
Impairment charge	9,604
Nonrecurring | Total Fair value
Non-Recurring Fair Value Measurements:
Long-lived assets held and used	32,718
Nonrecurring | Fair Value at Reporting Date Using Significant Unobservable Inputs (Level 3)
Non-Recurring Fair Value Measurements:
Long-lived assets held and used	$ 32,718
Number of long-lived held and used properties impaired	5