Segment Disclosure (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Consolidated net income
Operating revenues	$ 92,064	$ 72,416	$ 180,189	$ 139,235
Property and related expenses	26,540	21,383	52,763	39,407
Net Operating Income	65,524	51,033	127,426	99,828
Assets:
Land, buildings, and improvements, net	2,627,984		2,627,984		2,254,077
Undeveloped land and construction in progress	303,998		303,998		290,365
Total Assets	3,264,787		3,264,787		2,816,565
Reportable Segment-Office Properties [Member]
Consolidated net income
Operating revenues	84,560	64,718	165,379	124,321
Property and related expenses	24,729	19,503	47,639	35,757
Net Operating Income	59,831	45,215	117,740	88,564
Assets:
Land, buildings, and improvements, net	2,481,829		2,481,829		2,108,019
Undeveloped land and construction in progress	303,998		303,998		290,365
Total Assets	3,027,731		3,027,731		2,611,206
Non-Reportable Segment-Industrial Properties [Member]
Consolidated net income
Operating revenues	7,504	7,698	14,810	14,914
Property and related expenses	1,811	1,880	5,124	3,650
Net Operating Income	5,693	5,818	9,686	11,264
Assets:
Land, buildings, and improvements, net	146,155		146,155		146,058
Total Assets	160,172		160,172		159,612
Total Segments [Member]
Assets:
Total Assets	$ 3,187,903		$ 3,187,903		$ 2,770,818