Segment Reporting (Operating Activity Within Reportable Segments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total office revenues	$ 645,633	$ 540,975	$ 519,405
Office expenses	(214,546)	(186,556)	(181,160)
Total multifamily revenues	96,918	94,799	80,117
Multifamily expenses	(23,317)	(23,862)	(20,664)
Total profit from all segments	504,688	425,356	397,698
Office Segment [Member]
Segment Reporting Information [Line Items]
Total office revenues	645,633	540,975	519,405
Office expenses	(214,546)	(186,556)	(181,160)
Total profit from all segments	431,087	354,419	338,245
Multifamily Segment [Member]
Segment Reporting Information [Line Items]
Total multifamily revenues	96,918	94,799	80,117
Multifamily expenses	(23,317)	(23,862)	(20,664)
Total profit from all segments	$ 73,601	$ 70,937	$ 59,453