Fair Value Measurements (Assets and Liabilities Measured on a Recurring Basis) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	$ 833.7	$ 744.7	$ 833.7	$ 744.7		$ 742.0		$ 740.8
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	45.5		45.5			43.8
Foreign currency forward contracts receivable	0.0		0.0			0.0
Loans Receivable, Fair Value Disclosure	0.0		0.0			0.0
Deferred compensation plan assets	0.0		0.0			0.0
Mortgage banking derivative asset	0.0		0.0			0.0
Total assets at fair value	45.5		45.5			43.8
Foreign currency forward contracts payable	0.0		0.0			0.0
Deferred compensation plan liabilities	0.0		0.0			0.0
Business Combination, Contingent Consideration, Liability	0.0		0.0			0.0
Mortgage banking derivative liabilities	0.0		0.0			0.0
Total liabilities at fair value	0.0		0.0			0.0
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0.0		0.0			0.0
Foreign currency forward contracts receivable	6.3		6.3			4.9
Loans Receivable, Fair Value Disclosure	1,228.6		1,228.6			770.7
Deferred compensation plan assets	674.9		674.9			664.0
Mortgage banking derivative asset	0.0		0.0			0.0
Total assets at fair value	1,909.8		1,909.8			1,439.6
Foreign currency forward contracts payable	8.5		8.5			13.9
Deferred compensation plan liabilities	658.5		658.5			658.4
Business Combination, Contingent Consideration, Liability	0.0		0.0			0.0
Mortgage banking derivative liabilities	0.0		0.0			0.0
Total liabilities at fair value	667.0		667.0			672.3
Fair Value, Inputs, Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	19.9	31.7	19.9	31.7	$ 16.7	93.8	$ 18.9	10.3
Investments, Fair Value Disclosure	293.1	367.8	293.1	367.8	311.0	330.3	369.7	367.3
Foreign currency forward contracts receivable	0.0		0.0			0.0
Loans Receivable, Fair Value Disclosure	0.0		0.0			0.0
Deferred compensation plan assets	0.0		0.0			0.0
Mortgage banking derivative asset	54.2		54.2			161.1
Total assets at fair value	347.3		347.3			491.4
Foreign currency forward contracts payable	0.0		0.0			0.0
Deferred compensation plan liabilities	0.0		0.0			0.0
Business Combination, Contingent Consideration, Liability	19.1	53.7	19.1	53.7	$ 37.5	35.8	$ 46.7	$ 57.5
Mortgage banking derivative liabilities	34.3		34.3			67.3
Total liabilities at fair value	53.4		53.4			$ 103.1
Investments [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(25.3)	(9.3)	(45.2)	(10.4)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	1.4	(0.1)	2.1	(0.3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	6.0	2.3	6.0	2.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0.0	0.0	(0.1)	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0.0	5.2	0.0	8.4
Derivative [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	9.7	19.5	17.0	28.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	34.9	22.3	64.9	46.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(41.4)	(29.0)	(155.8)	(52.9)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		0.0	0.0	0.0
Earn-out Liabilities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(7.4)	(2.1)	(3.5)	(12.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0.3	0.0	0.4	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0.2	11.0	0.2	11.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(12.0)	(1.7)	(12.1)	(2.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	$ 0.5	$ (0.2)	$ (1.7)	$ (0.2)

JONES LANG LASALLE INC 10-Q