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Commitments and Contingencies (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Jun. 30, 2025	Dec. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
Loss Contingency, Receivable				$ 0.5
Loan subject to loss-sharing arrangements, aggregate unpaid principal amount	$ 24,700.0			23,000.0
Loan loss accrual	29.9			30.0
Loan loss guarantee reserve	29.7			28.5
Level of risk retained (per claim)	10.0
Loss Contingency Accrual [Roll Forward]
Loss Contingency, Receivable				$ 0.5
Loan loss incurred [Abstract]			$ 20.6
Insurance Claims [Member]
Loss Contingency Accrual [Roll Forward]
Loss Contingency Accrual	4.2	$ 9.4
Loss Contingency Accrual, Provision	3.9	0.3
Loss Contingency Accrual, Provision Adjustment	0.3	1.2
Loss Contingency Accrual, Payments	(6.1)	(7.2)
Loss Contingency Accrual	$ 2.3	$ 3.7

X

- Definition

The per claim amount needed to reflect the estimated ultimate cost of settling claims relating to casualty insurance insured events that have occurred on or before a particular date (ordinarily, the balance sheet date) and the amount needed to provide for the estimated ultimate cost required to investigate and settle claims relating to insured events that have occurred on or before a particular date (ordinarily, the balance sheet date), whether or not reported to the insurer at that date.

+ References

No definition available.

+ Details

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Balance Type:	credit
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X

- Definition

Loan loss expense

+ References

No definition available.

+ Details

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X

- Definition

Loan loss accrual

+ References

No definition available.

+ Details

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Namespace Prefix:	jll_
Data Type:	xbrli:monetaryItemType
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X

- Definition

contingent reserve for loss sharing program

+ References

No definition available.

+ Details

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X

- Definition

Loan subject to loss-sharing arrangements, aggregate unpaid principal amount

+ References

No definition available.

+ Details

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- Definition

Loss Contingency Accrual, Provision Adjustment

+ References

No definition available.

+ Details

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Namespace Prefix:	jll_
Data Type:	xbrli:monetaryItemType
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- References

No definition available.

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X

- Definition

Amount of loss contingency liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-1

Reference 2: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-10

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X

- Definition

Amount of cash outflow reducing loss contingency liability.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-1

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- Definition

Amount charged against operating income increasing loss contingency liability, after adjustments to reduce previously estimated charges.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-1

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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X

- Definition

Amount of receivable related to a loss contingency accrual. For example, an insurance recovery receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-3

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