Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Rental and other revenues		$ 167,646	$ 166,269	$ 166,860	$ 164,859	$ 160,126	$ 150,766	$ 148,543	$ 145,236	$ 665,634	$ 604,671	$ 555,871
Total net operating income										434,549	388,730	349,987
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization										(220,140)	(201,918)	(180,637)
Impairments of real estate assets										0	0	(588)
General and administrative expenses										(38,153)	(37,642)	(35,258)
Interest expense										(76,648)	(86,052)	(85,127)
Other income										2,338	1,726	2,431
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates										101,946	64,844	50,808
Total Assets		4,561,050				4,485,631				4,561,050	4,485,631
Office Total Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										651,511	589,946	538,777
Total net operating income										424,731	378,632	338,698
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		4,340,446				4,080,264				4,340,446	4,080,264
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										134,601	108,590	96,075
Total net operating income										84,733	67,094	58,180
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		1,039,519				1,013,226				1,039,519	1,013,226
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										20,522	21,251	25,018
Total net operating income										12,781	13,395	15,784
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		127,887				131,027				127,887	131,027
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										48,251	47,137	41,016
Total net operating income										30,038	29,534	24,376
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		271,115				277,866				271,115	277,866
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										95,912	88,310	80,722
Total net operating income										68,678	62,387	55,354
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		714,905				591,111				714,905	591,111
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										46,260	44,621	36,574
Total net operating income										26,525	25,524	21,286
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		307,021				303,843				307,021	303,843
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										58,789	59,392	56,692
Total net operating income										34,175	34,348	31,505
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		353,816				339,186				353,816	339,186
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										112,958	102,841	87,428
Total net operating income										80,803	72,981	61,317
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		760,607				663,617				760,607	663,617
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										44,315	42,089	45,559
Total net operating income										30,505	27,922	30,021
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		212,508				211,574				212,508	211,574
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										89,903	75,715	69,693
Total net operating income										56,493	45,447	40,875
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets		553,068				548,814				553,068	548,814
Other Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues										14,123	14,725	17,094
Total net operating income										9,818	10,098	$ 11,289
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	[1]	$ 220,604				$ 405,367				$ 220,604	$ 405,367

[1] Includes the Plaza assets, which were included in real estate and other assets, net, held for sale on our Consolidated Balance Sheets at December 31, 2015.