Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended																12 Months Ended
		Dec. 31, 2015		Sep. 30, 2015	[1]	Jun. 30, 2015	[1]	Mar. 31, 2015	[1]	Dec. 31, 2014		Sep. 30, 2014	[1]	Jun. 30, 2014	[1]	Mar. 31, 2014	[1]	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Rental and other revenues		$ 160,126	[1]	$ 150,766		$ 148,543		$ 145,236		$ 140,724	[1]	$ 139,370		$ 139,956		$ 135,821		$ 604,671	$ 555,871	$ 505,008
Total net operating income																		388,730	349,987	320,821
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization																		(201,918)	(180,637)	(162,937)
Impairments of real estate assets																		0	(588)	0
General and administrative expense																		(37,642)	(35,258)	(36,381)
Interest expense																		(86,052)	(85,127)	(91,886)
Other income																		1,726	2,431	3,312
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates																		64,844	50,808	32,929
Total Assets		4,493,432								3,999,123								4,493,432	3,999,123
Office Total Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		589,946	540,917	489,634
Total net operating income																		378,632	339,848	310,346
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		4,080,264								3,573,696								4,080,264	3,573,696
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		108,590	96,075	80,330
Total net operating income																		67,094	58,180	49,650
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		1,013,226								700,085								1,013,226	700,085
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		21,251	25,018	26,047
Total net operating income																		13,395	15,784	16,788
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		131,027								139,479								131,027	139,479
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		0	2,140	3,399
Total net operating income																		0	1,150	1,893
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		47,137	41,016	38,369
Total net operating income																		29,534	24,376	22,133
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		277,866								280,186								277,866	280,186
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		88,310	80,722	62,054
Total net operating income																		62,387	55,354	42,598
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		591,111								527,317								591,111	527,317
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		44,621	36,574	21,798
Total net operating income																		25,524	21,286	12,048
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		303,843								291,611								303,843	291,611
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		59,392	56,692	56,125
Total net operating income																		34,348	31,505	31,134
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		339,186								334,539								339,186	334,539
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		102,841	87,428	85,417
Total net operating income																		72,981	61,317	60,075
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		663,617								669,450								663,617	669,450
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		42,089	45,559	47,576
Total net operating income																		27,922	30,021	32,454
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		211,574								215,987								211,574	215,987
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		75,715	69,693	68,519
Total net operating income																		45,447	40,875	41,573
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		548,814								415,042								548,814	415,042
Other Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues																		14,725	14,954	15,374
Total net operating income																		10,098	10,139	$ 10,475
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	[2]	$ 413,168								$ 425,427								$ 413,168	$ 425,427

[1] As a result of discontinued operations, the amounts presented may not equal the amounts previously reported in the most recent Form 10-Qs or prior 10-K for each period. Below is a reconciliation to the amounts previously reported: Quarter Ended March 31, June 30, September 30, 2015 2015 2015Rental and other revenues, as reported$157,310 $161,136 $163,736Discontinued operations(12,074) (12,593) (12,970)Rental and other revenues, as adjusted$145,236 $148,543 $150,766Income from continuing operations, as reported$20,835 $26,891 $31,617Discontinued operations(3,915) (4,670) (4,265)Income from continuing operations, as adjusted$16,920 $22,221 $27,352Income from discontinued operations, as reported$— $— $—Additional discontinued operations from properties sold subsequent to the respective reporting period3,915 4,670 4,265Income from discontinued operations, as adjusted$3,915 $4,670 $4,265 Quarter Ended March 31, June 30, September 30, December 31, 2014 2014 2014 2014Rental and other revenues, as reported$148,453 $152,722 $152,629 $154,664Discontinued operations(12,632) (12,766) (13,259) (13,940)Rental and other revenues, as adjusted$135,821 $139,956 $139,370 $140,724Income from continuing operations, as reported$13,192 $24,512 $54,299 $23,585Discontinued operations(4,213) (4,138) (4,816) (5,434)Income from continuing operations, as adjusted$8,979 $20,374 $49,483 $18,151Income from discontinued operations, as reported$384 $— $— $—Additional discontinued operations from properties sold subsequent to the respective reporting period4,213 4,138 4,816 5,434Income from discontinued operations, as adjusted$4,597 $4,138 $4,816 $5,434

[2] Includes the Plaza assets, which are included in real estate and other assets, net, held for sale on our Consolidated Balance Sheets.