Discontinued Operations - Income from Office Property Disposition and Held for Sale (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Book Value of Real Estate	$ 2,968,222		$ 2,968,222		$ 3,024,271
Gain on Sale	$ 0	$ 0	$ 0	$ 10,463
2014 Dispositions
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price		26,681		26,681
Net Book Value of Real Estate		16,218		16,218
Gain on Sale		$ 10,463		$ 10,463
Woodbranch Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Location		Houston, TX		Houston, TX
Date of Sale		Jan. 14, 2014		Jan. 14, 2014
Net Sales Price		$ 14,424		$ 14,424
Net Book Value of Real Estate		4,450		4,450
Gain on Sale		$ 9,974		$ 9,974
Mesa Corporate Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Location		Phoenix, AZ		Phoenix, AZ
Date of Sale		Jan. 31, 2014		Jan. 31, 2014
Net Sales Price		$ 12,257		$ 12,257
Net Book Value of Real Estate		11,768		11,768
Gain on Sale		$ 489		$ 489