Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Rental and other revenues	$ 154,664	$ 152,629	$ 152,722	$ 148,453	$ 148,988	$ 144,827	$ 132,618	$ 130,377	$ 608,468	$ 556,810	$ 485,046
Total net operating income									382,964	353,466	308,302
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Depreciation and amortization									(196,023)	(176,957)	(146,357)
Impairments of real estate assets									(588)	0	0
General and administrative expense									(36,223)	(37,193)	(37,377)
Interest expense									(85,852)	(92,703)	(96,114)
Other income									5,131	6,398	6,380
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates									69,409	53,011	34,834
Total Assets	4,004,909				3,807,101				4,004,909	3,807,101
Office Total Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									557,475	505,937	434,531
Total net operating income									350,617	321,040	276,446
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	3,661,786				3,425,874				3,661,786	3,425,874
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									96,075	80,330	60,474
Total net operating income									58,180	49,650	38,186
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	700,085				699,263				700,085	699,263
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									25,018	26,047	19,489
Total net operating income									15,784	16,788	12,282
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	139,479				164,885				139,479	164,885
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									2,140	3,399	3,269
Total net operating income									1,150	1,893	1,701
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	0				15,890				0	15,890
Office Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									16,558	16,303	14,995
Total net operating income									10,769	10,694	9,509
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	82,304				83,124				82,304	83,124
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									41,016	38,369	36,812
Total net operating income									24,376	22,133	21,831
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	280,186				249,479				280,186	249,479
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									80,722	62,054	56,512
Total net operating income									55,354	42,598	38,801
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	527,317				490,887				527,317	490,887
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									36,574	21,798	9,052
Total net operating income									21,286	12,048	4,334
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	291,611				226,314				291,611	226,314
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									56,692	56,125	38,776
Total net operating income									31,505	31,134	19,530
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	334,539				335,798				334,539	335,798
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									87,428	85,417	81,581
Total net operating income									61,317	60,075	56,584
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	669,450				494,208				669,450	494,208
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									45,559	47,576	47,284
Total net operating income									30,021	32,454	32,382
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	215,987				241,739				215,987	241,739
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									69,693	68,519	66,287
Total net operating income									40,875	41,573	41,306
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	420,828				424,287				420,828	424,287
Industrial Total Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									12,212	12,997	13,324
Total net operating income									8,714	9,429	9,610
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	79,534				100,772				79,534	100,772
Industrial Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									657	827	812
Total net operating income									364	492	468
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	12,063				25,936				12,063	25,936
Industrial Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									11,555	12,170	12,512
Total net operating income									8,350	8,937	9,142
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	67,471				74,836				67,471	74,836
Total Retail Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									38,781	37,876	37,191
Total net operating income									23,724	23,074	22,510
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	159,390				161,779				159,390	161,779
Retail Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									38,781	37,876	37,191
Total net operating income									23,724	23,074	22,510
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	159,390				161,779				159,390	161,779
Total Residential Segment [Member]
Segment Reporting Information [Line Items]
Total net operating income									0	0	(178)
Residential Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total net operating income									0	0	(178)
Total Corporate and Other Segment [Member]
Segment Reporting Information [Line Items]
Total net operating income									(91)	(77)	(86)
Reconciliation to income from continuing operations before disposition of property, condominiums and investments in unconsolidated affiliates and equity in earnings of unconsolidated affiliates:
Total Assets	$ 104,199				$ 118,676				$ 104,199	$ 118,676