Notes Payable - Terms of Notes Payable (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Notes payable, gross	$ 2,212,863	$ 2,160,293
Unamortized premium	6,658	7,574
Unamortized loan costs	(4,829)	(5,148)
Total Notes Payable	$ 2,214,692	2,162,719
Weighted average maturity of notes payable	4 years 3 months 18 days
Unsecured Notes
Debt Instrument [Line Items]
Notes payable, gross	$ 1,539,000	1,482,400
Credit Facility, Unsecured
Debt Instrument [Line Items]
Interest Rate	1.16%
Notes payable, gross	$ 289,000	232,400
Term Loan, Unsecured
Debt Instrument [Line Items]
Interest Rate	1.31%
Notes payable, gross	$ 250,000	250,000
Senior Notes, Unsecured \| 3.95% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.95%
Notes payable, gross	$ 275,000	275,000
Senior Notes, Unsecured \| 3.91% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.91%
Notes payable, gross	$ 250,000	250,000
Senior Notes, Unsecured \| 3.86% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.86%
Notes payable, gross	$ 250,000	250,000
Senior Notes, Unsecured \| 3.78% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.78%
Notes payable, gross	$ 125,000	125,000
Senior Notes, Unsecured \| 4.09% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	4.09%
Notes payable, gross	$ 100,000	100,000
Secured Mortgage Notes
Debt Instrument [Line Items]
Notes payable, gross	$ 673,863	677,893
Secured Mortgage Notes \| Fifth Third Center
Debt Instrument [Line Items]
Interest Rate	3.37%
Notes payable, gross	$ 136,222	137,057
Secured Mortgage Notes \| Terminus 100
Debt Instrument [Line Items]
Interest Rate	5.25%
Notes payable, gross	$ 114,183	114,997
Secured Mortgage Notes \| Colorado Tower
Debt Instrument [Line Items]
Interest Rate	3.45%
Notes payable, gross	$ 114,040	114,660
Secured Mortgage Notes \| Promenade
Debt Instrument [Line Items]
Interest Rate	4.27%
Notes payable, gross	$ 91,722	92,593
Secured Mortgage Notes \| 816 Congress
Debt Instrument [Line Items]
Interest Rate	3.75%
Notes payable, gross	$ 77,784	78,232
Secured Mortgage Notes \| Terminus 200
Debt Instrument [Line Items]
Interest Rate	3.79%
Notes payable, gross	$ 73,912	74,354
Secured Mortgage Notes \| Legacy Union One
Debt Instrument [Line Items]
Interest Rate	4.24%
Notes payable, gross	$ 66,000	$ 66,000

Debt Instrument, Weighted Average Maturity Period

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
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-Name Accounting Standards Codification
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-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

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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.

+ Details

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- Definition

Amount, after accumulated amortization, of debt premium.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
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-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28541-108399

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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

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-Publisher FASB
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- Definition

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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
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