Segment Reporting (Operating Activity Within Reportable Segments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information [Line Items]
Rental revenue	$ 129,087	$ 126,423
Rental expense	(43,356)	(41,309)
Rental revenue	19,789	19,035
Rental expense	(5,133)	(5,009)
Segment profit	100,387	99,140
Office Segment [Member]
Segment Reporting Information [Line Items]
Rental revenue	129,087	126,423
Rental expense	(43,356)	(41,309)
Segment profit	85,731	85,114
Multifamily Segment [Member]
Segment Reporting Information [Line Items]
Rental revenue	19,789	19,035
Rental expense	(5,133)	(5,009)
Segment profit	$ 14,656	$ 14,026