Capital and Financing Transactions - Narrative (Details)													3 Months Ended								12 Months Ended
	Dec. 22, 2015 USD ($)	Jul. 16, 2015 USD ($)	Jun. 26, 2015 USD ($)	Apr. 08, 2015 USD ($)	Apr. 06, 2015 USD ($)	Apr. 03, 2015 USD ($)	Mar. 05, 2015 USD ($)	Jan. 27, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jul. 02, 2014 USD ($)	Apr. 08, 2014 USD ($)	Apr. 01, 2014 USD ($) draw	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Oct. 01, 2015 USD ($)	Nov. 17, 2014	Apr. 14, 2014 USD ($)
Notes Payable [Abstract]
Outstanding balance of credit facilities									$ 481,500,000				$ 550,000,000				$ 481,500,000				$ 550,000,000	$ 481,500,000
Line of Credit Facility [Line Items]
Closing amount on construction loan													31,300,000								31,300,000
Gain (loss) on extinguishment of debt													(520,000)	$ (702,000)	$ (4,919,000)	$ 79,000	(2,066,000)	$ 0	$ (339,000)	$ 0	(6,062,000)	(2,405,000)	$ 0
Repayments of secured debt																					180,315,000	53,402,000	$ 73,385,000
San Jacinto Center and One Congress Plaza | CalSTRS
Line of Credit Facility [Line Items]
Ownership interest acquired (as a percent)																									60.00%
Ownership of property after acquisition (as a percent)																									100.00%
One Orlando Centre | Florida
Line of Credit Facility [Line Items]
Existing first mortgage amount																										$ 68,300,000
Ownership interest acquired (as a percent)																										100.00%
One Orlando Centre | Florida | Second Mortgage
Line of Credit Facility [Line Items]
Mortgage notes payable																										$ 15,300,000.0
One Orlando Centre | Florida | First Mortgage
Line of Credit Facility [Line Items]
Mortgage notes payable																										$ 54,000,000.0
One Congress Plaza | CalSTRS
Line of Credit Facility [Line Items]
Existing first mortgage amount									$ 128,000,000								$ 128,000,000					$ 128,000,000
Fixed interest rate									6.10%								6.10%					6.10%
San Jacinto Center | CalSTRS
Line of Credit Facility [Line Items]
Existing first mortgage amount									$ 101,000,000								$ 101,000,000					$ 101,000,000
Fixed interest rate									6.00%								6.00%					6.00%
Interest Rate Swap
Line of Credit Facility [Line Items]
Floating-to-fixed interest rate swap											$ 33,900,000
Liability value											1,900,000
Notional amount											100,000,000
Interest Rate Swap, 2.6% | Cash Flow Hedging | Designated as Hedging Instrument
Line of Credit Facility [Line Items]
Notional amount												$ 100,000,000
Fixed rate												2.60%
Mortgages | Parent Company | 245 Riverside
Line of Credit Facility [Line Items]
Prepayment fee incurred		$ 210,000
Mortgages | Parent Company | Two Ravinia Drive
Line of Credit Facility [Line Items]
Prepayment fee incurred				$ 525,000
Two Buckhead Plaza
Line of Credit Facility [Line Items]
Mortgage notes payable																								$ 52,000,000.0
Fixed interest rate																								6.43%
245 Riverside | Mortgages | Fund II
Line of Credit Facility [Line Items]
Repayments of secured debt		9,100,000
Prepayment fee incurred		$ 702,000
Bank of America Center | Mortgages
Line of Credit Facility [Line Items]
Gain (loss) on extinguishment of debt											(339,000)
One Orlando Centre
Line of Credit Facility [Line Items]
Annual return (as a percent)																										12.00%
Percentage of equity to the company																										60.00%
Percentage of equity to the lender																										40.00%
Distribution preference, fifth, remaining proceeds																										100.00%
One Orlando Centre | Florida | Second Mortgage
Line of Credit Facility [Line Items]
Fair value of subordinated note													0								0
Raymond James Tower | Mortgages
Line of Credit Facility [Line Items]
Principal balance									$ 7,900,000
Gain (loss) on extinguishment of debt									(2,100,000)
Westshore Corporate Center | Mortgages
Line of Credit Facility [Line Items]
Principal balance							$ 14,000,000
Gain (loss) on extinguishment of debt							$ 79,000
Hillsboro I-V, Peachtree Dunwoody, One Commerce Green and Comerica Bank Building | Mortgages
Line of Credit Facility [Line Items]
Repayments of secured debt						$ 68,000,000
Prepayment fee incurred						$ 3,000,000
3350 Peachtree | Mortgages
Line of Credit Facility [Line Items]
Repayments of secured debt					$ 31,900,000
Prepayment fee incurred					$ 319,000
Two Ravinia Drive | Mortgages | Fund II
Line of Credit Facility [Line Items]
Repayments of secured debt				22,100,000
Prepayment fee incurred				$ 1,800,000
5300 Memorial and Town & Country | Mortgages
Line of Credit Facility [Line Items]
Repayments of secured debt	$ 17,100,000
Prepayment fee incurred	$ 503,000
First Mortgage | Bank of America Center
Line of Credit Facility [Line Items]
Repayments of debt											$ 33,900,000
Revolving Credit Facility | Amended Working Capital Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity												$ 10,000,000.0
Unsecured Term Loan
Notes Payable [Abstract]
Outstanding balance of credit facilities													200,000,000								200,000,000
Unsecured Term Loan $120 million
Line of Credit Facility [Line Items]
Mortgage notes payable												120,000,000.0
Unsecured Revolving Credit Facility
Notes Payable [Abstract]
Outstanding balance of credit facilities									131,500,000				$ 0				$ 131,500,000				$ 0	$ 131,500,000
Line of Credit Facility [Line Items]
Mortgage notes payable												215,000,000.0
Unsecured Term Loan $125 million
Line of Credit Facility [Line Items]
Mortgage notes payable												125,000,000.0
Secured Debt | Hayden Ferry Lakeside III
Line of Credit Facility [Line Items]
Closing amount on construction loan										$ 43,000,000
Percentage of total estimated cost of development										62.50%
Recourse loan (as a percent)										35.00%
Secured Debt | Hayden Ferry Loan, Pre-stabilization | Hayden Ferry Lakeside III | One-Month LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)										1.80%
Secured Debt | Hayden Ferry Loan, Post-stabilization | Hayden Ferry Lakeside III | One-Month LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)										1.60%
Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity												$ 250,000,000.0
Term of extension option on debt instrument												1 year
Period end interest rate (in percent)													1.49%								1.49%
Line of Credit | Revolving Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Interest rate during the period																					1.30%
Line of Credit | Unsecured Term Loan
Line of Credit Facility [Line Items]
Term of debt instrument			5 years																		5 years
Mortgage notes payable			$ 200,000,000.0										$ 200,000,000.0								$ 200,000,000.0
Unsecured term loan			$ 400,000,000.0
Line of Credit | Unsecured Term Loan | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)			1.35%
Line of Credit | Unsecured Term Loan | Minimum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)			0.90%
Line of Credit | Unsecured Term Loan | Maximum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)			1.75%
Line of Credit | Unsecured Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity								$ 450,000,000.0	250,000,000.0				450,000,000.0				250,000,000.0				$ 450,000,000.0	$ 250,000,000.0
Increase in amount of facility								$ 200,000,000.0
Unsecured Debt | Five-year Term Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity												$ 250,000,000.0
Term of debt instrument												5 years									5 years	5 years
Mortgage notes payable									250,000,000.0				$ 250,000,000.0				250,000,000.0				$ 250,000,000.0	$ 250,000,000.0
Period end interest rate (in percent)													2.56%								2.56%
Unsecured Debt | Five-year Term Loan | LIBOR
Line of Credit Facility [Line Items]
Interest rate during the period																					1.40%
Unsecured Debt | Seven-year Term Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity												$ 100,000,000.0
Term of debt instrument												7 years									7 years	7 years
Mortgage notes payable									100,000,000.0				$ 100,000,000.0				100,000,000.0				$ 100,000,000.0	$ 100,000,000.0
Number of draw permitted on line of credit over 12-month period | draw												2
Number of draw permitted period												12 months
Period end interest rate (in percent)												4.31%	4.41%								4.41%
Unsecured Debt | Seven-year Term Loan | LIBOR
Line of Credit Facility [Line Items]
Interest rate during the period												1.75%									1.85%
Unsecured Debt | Seven-year Term Loan | Minimum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)												1.75%
Unsecured Debt | Seven-year Term Loan | Maximum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)												2.30%
Mortgages
Line of Credit Facility [Line Items]
Net book value									$ 2,000,000,000				$ 1,900,000,000				$ 2,000,000,000				$ 1,900,000,000	$ 2,000,000,000