Debt Financing Debt Financing Schedule of Long Term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt, principal outstanding	$ 1,936,813	$ 1,840,277
Unamortized Deferred Financing Fees	(4,638)	(5,324)
Notes Payable, net	$ 1,932,175	1,834,953
JPMorgan Chase Revolving Credit Facility - Revolving Loan
Debt Instrument [Line Items]
Credit facility, interest rate (percent)	3.37%
Credit facility, principal outstanding	$ 208,000	99,000
JPMorgan Chase Revolving Credit Facility - Term Loan
Debt Instrument [Line Items]
Credit facility, interest rate (percent)	3.33%
Credit facility, principal outstanding	$ 495,000	495,000
100 Brookes \| Secured Debt
Debt Instrument [Line Items]
Debt, principal outstanding		28,098
Poland Logistics Portfolio \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	2.00%
Debt, principal outstanding	$ 72,851	71,183
Minneapolis Retail Center \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	3.50%
Debt, principal outstanding	$ 65,500	65,500
825 Ann \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	2.53%
Debt, principal outstanding	$ 62,218	63,247
465 Victoria \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	3.06%
Debt, principal outstanding	$ 40,853	41,528
New City \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	2.30%
Debt, principal outstanding	$ 82,546	80,831
One Westferry Circus \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	3.30%
Debt, principal outstanding	$ 67,238	64,757
The Campus at Playa Vista \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	3.07%
Debt, principal outstanding	$ 150,000	150,000
Perspective Défense \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	2.17%
Debt, principal outstanding	$ 86,247	83,853
Fiege Mega Centre \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	1.37%
Debt, principal outstanding	$ 27,574	26,898
25 Cabot Square \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	3.50%
Debt, principal outstanding	$ 173,349	166,951
Simon Hegele Logistics \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	1.90%
Debt, principal outstanding	$ 42,988	41,904
818 Bourke \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	2.58%
Debt, principal outstanding	$ 64,495	65,562
The Summit \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	3.22%
Debt, principal outstanding	$ 170,000	170,000
Harder Logistics Portfolio \| Secured Debt
Debt Instrument [Line Items]
Effective interest rate (percent)	0.95%
Debt, principal outstanding	$ 83,057	81,068
WaterWall Place Loan \| Construction Loan Payable
Debt Instrument [Line Items]
Effective interest rate (percent)	3.27%
Debt, principal outstanding	$ 44,897	$ 44,897

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

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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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.

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No definition available.

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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
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

The effective interest rate at the end of the reporting period.

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- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

+ References

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(a)(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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