Notes Payable (Terms of Notes Payable) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Notes Payable, Gross	$ 1,855,153	$ 1,067,362
Unamortized premium	2,335	0
Unamortized loan costs	(6,671)	(4,792)
Total Notes Payable	$ 1,850,817	1,062,570
Weighted average maturity of notes payable	6 years 3 months 18 days
Senior Notes, Unsecured \| 3.95% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.95%
Notes Payable, Gross	$ 275,000	0
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	0
Senior Notes, Unsecured \| 3.78% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.78%
Notes Payable, Gross	$ 125,000	0
Senior Notes, Unsecured \| 4.09% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	4.09%
Notes Payable, Gross	$ 100,000	100,000
Term Loan, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.22%
Notes Payable, Gross	$ 250,000	250,000
Mortgage Debt \| Fifth Third Center
Debt Instrument [Line Items]
Interest Rate	3.37%
Notes Payable, Gross	$ 141,131	143,497
Mortgage Debt \| Colorado Tower
Debt Instrument [Line Items]
Interest Rate	3.45%
Notes Payable, Gross	$ 117,677	119,427
Mortgage Debt \| Promenade
Debt Instrument [Line Items]
Interest Rate	4.27%
Notes Payable, Gross	$ 96,813	99,238
Mortgage Debt \| 816 Congress
Debt Instrument [Line Items]
Interest Rate	3.75%
Notes Payable, Gross	$ 80,415	81,676
Mortgage Debt \| Legacy Union One
Debt Instrument [Line Items]
Interest Rate	4.24%
Notes Payable, Gross	$ 66,000	0
Mortgage Debt \| Meridian Mark Plaza
Debt Instrument [Line Items]
Interest Rate	6.00%
Notes Payable, Gross	$ 23,117	23,524
Credit Facility, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.07%
Notes Payable, Gross	$ 80,000	$ 0

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=109500613&loc=SL6031897-161870

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+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

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Balance Type:	na
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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

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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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- 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
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