Notes Payable (Terms of Notes Payable) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Notes Payable, Gross	$ 1,100,903	$ 1,378,676
Unamortized premium, net	270	6,792
Unamortized loan costs	(5,996)	(4,548)
Total Notes Payable	$ 1,095,177	1,380,920
Term Loan, Unsecured
Debt Instrument [Line Items]
Interest Rate	2.43%
Notes Payable, Gross	$ 250,000	250,000
Mortgage Debt \| Fifth Third Center
Debt Instrument [Line Items]
Interest Rate	3.37%
Notes Payable, Gross	$ 147,306	149,516
Mortgage Debt \| Colorado Tower
Debt Instrument [Line Items]
Interest Rate	3.45%
Notes Payable, Gross	$ 120,000	120,000
Mortgage Debt \| Promenade
Debt Instrument [Line Items]
Interest Rate	4.27%
Notes Payable, Gross	$ 103,113	105,342
Mortgage Debt \| 816 Congress
Debt Instrument [Line Items]
Interest Rate	3.75%
Notes Payable, Gross	$ 83,702	84,872
Mortgage Debt \| Meridian Mark Plaza
Debt Instrument [Line Items]
Interest Rate	6.00%
Notes Payable, Gross	$ 24,162	24,522
Mortgage Debt \| The Pointe
Debt Instrument [Line Items]
Interest Rate	4.01%
Notes Payable, Gross	$ 22,620	$ 22,945
Mortgage Debt \| 3344 Peachtree
Debt Instrument [Line Items]
Interest Rate		4.75%
Notes Payable, Gross	0	$ 78,971
Mortgage Debt \| One Eleven Congress
Debt Instrument [Line Items]
Interest Rate		6.08%
Notes Payable, Gross	0	$ 128,000
Mortgage Debt \| The ACS Center
Debt Instrument [Line Items]
Interest Rate		6.45%
Notes Payable, Gross	0	$ 127,508
Mortgage Debt \| San Jacinto Center
Debt Instrument [Line Items]
Interest Rate		6.048%
Notes Payable, Gross	0	$ 101,000
Mortgage Debt \| Two Buckhead Plaza
Debt Instrument [Line Items]
Interest Rate		6.43%
Notes Payable, Gross	$ 0	$ 52,000
Senior Notes, Unsecured \| 3.91% Senior Note, unsecured
Debt Instrument [Line Items]
Interest Rate	3.91%
Notes Payable, Gross	$ 250,000	0
Senior Notes, Unsecured \| 4.09% Senior Note, unsecured
Debt Instrument [Line Items]
Interest Rate	4.09%
Notes Payable, Gross	$ 100,000	$ 0
Credit Facility, Unsecured
Debt Instrument [Line Items]
Interest Rate		2.33%
Notes Payable, Gross	$ 0	$ 134,000

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount, after accumulated amortization, of debt premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Name Accounting Standards Codification
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