Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Rental and other revenues	$ 175,861	$ 180,185	$ 177,283	$ 169,408	$ 167,646	$ 166,269	$ 166,860	$ 164,859	$ 702,737	$ 665,634	$ 604,671
Total net operating income									465,849	434,549	388,730
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization									(227,832)	(220,140)	(201,918)
Impairments of real estate assets									(1,445)	0	0
General and administrative expenses									(39,648)	(38,153)	(37,642)
Interest expense									(69,105)	(76,648)	(86,052)
Other income									2,283	2,338	1,726
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates									130,102	101,946	64,844
Total Assets	4,623,791				4,561,050				4,623,791	4,561,050
Office Total Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									689,788	651,511	589,946
Total net operating income									456,628	424,731	378,632
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	4,391,387				4,340,446				4,391,387	4,340,446
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									140,323	134,601	108,590
Total net operating income									89,575	84,733	67,094
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	1,049,100				1,039,519				1,049,100	1,039,519
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									21,453	20,522	21,251
Total net operating income									13,612	12,781	13,395
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	134,194				127,887				134,194	127,887
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									45,430	48,251	47,137
Total net operating income									28,128	30,038	29,534
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	218,088				271,115				218,088	271,115
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									111,506	95,912	88,310
Total net operating income									81,204	68,678	62,387
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	806,725				714,905				806,725	714,905
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									51,236	46,260	44,621
Total net operating income									30,526	26,525	25,524
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	306,970				307,021				306,970	307,021
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									59,103	58,789	59,392
Total net operating income									34,784	34,175	34,348
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	334,136				353,816				334,136	353,816
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									119,254	112,958	102,841
Total net operating income									86,475	80,803	72,981
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	762,331				760,607				762,331	760,607
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									43,959	44,315	42,089
Total net operating income									29,946	30,505	27,922
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	229,468				212,508				229,468	212,508
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									97,524	89,903	75,715
Total net operating income									62,378	56,493	45,447
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	550,375				553,068				550,375	553,068
Other Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									12,949	14,123	14,725
Total net operating income									9,221	9,818	$ 10,098
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	$ 232,404				$ 220,604				$ 232,404	$ 220,604