Segment Disclosure (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Consolidated net income
Operating revenues	$ 99,410	[1]	$ 83,773	[1]
Property and related expenses	26,728		25,764
Net Operating Income	72,682		58,009
Reconciliation to Consolidated Net Income:
Total Net Operating Income for segments	72,682		58,009
Unallocated Expenses Income [Abstract]
General and administrative expenses	(8,767)		(6,560)
Acquisition-related expenses	(1,528)		(472)
Depreciation and amortization	(36,746)		(28,441)
Interest income and other net investment gains	484		184
Interest expense	(21,163)		(20,876)
INCOME FROM CONTINUING OPERATIONS	4,962		1,844
Income from discontinued operations	73,709	[2]	3,023	[2],[3]
Net income	78,671		4,867
Reportable Segment - Office Properties [Member]
Segment Reporting Information [Line Items]
Business Segment	1
Consolidated net income
Operating revenues	92,680	[1]	77,264	[1]
Property and related expenses	24,799		22,494
Net Operating Income	67,881		54,770
Reconciliation to Consolidated Net Income:
Total Net Operating Income for segments	67,881		54,770
Unallocated Expenses Income [Abstract]
Income from discontinued operations	73,709		2,309
Non-Reportable Segment - Industrial Properties [Member]
Segment Reporting Information [Line Items]
Business Segment	1
Consolidated net income
Operating revenues	6,730	[1]	6,509	[1]
Property and related expenses	1,929		3,270
Net Operating Income	4,801		3,239
Reconciliation to Consolidated Net Income:
Total Net Operating Income for segments	4,801		3,239
Unallocated Expenses Income [Abstract]
Income from discontinued operations	$ 0		$ 714

[1] All operating revenues are comprised of amounts received from third-party tenants.

[2] See Note 14 for the breakdown of income from discontinued operations by segment.

[3] Includes two office and one industrial buildings encompassing 282,611 rentable square feet that were disposed of in 2011.