Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 25.2	$ 26.9	$ 22.8
Stock Options [Member] \| U.K. Save As You Earn (SAYE) Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant (in shares)	240,000
Allocated share-based compensation expense	$ 1.4	$ 1.4	2.1
Other Stock Compensation Programs [Abstract]
Options granted (in shares)	85,000	0
Exercise price - options granted (in dollars per share)	$ 90.97	$ 0.00
Options exercised (in shares)	28,000	20,000
Weighted average exercise price (in dollars per share)	$ 121.70	$ 70.37
Stock Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant (in shares)	400,000
Stock Compensation Plan [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 23.8	$ 25.5	$ 20.7
Restricted Stock Units (RSUs) [Member]
Restricted stock unit activity [Roll Forward]
Unvested at beginning of period (in shares)	750,900	706,000	745,300
Granted (in shares)	188,400	299,300	186,300
Vested (in shares)	(186,500)	(203,600)	(196,400)
Forfeited (in shares)	(25,100)	(50,800)	(29,200)
Unvested at end of period (in shares)	727,700	750,900	706,000
Restricted stock unit activity, additional disclosures [Abstract]
Weighted average grant date fair value, beginning of period (in dollars per share)	$ 113.97	$ 111.78	$ 90.43
Weighted average grant date fair value, granted (in dollars per share)	119.08	107.74	159.30
Weighted average grant date fair value, vested (in dollars per share)	99.23	96.37	78.45
Weighted average grant date fair value, forfeited (in dollars per share)	117.07	117.48	94.20
Weighted average grant date fair value, end of period (in dollars per share)	$ 118.96	$ 113.97	$ 111.78
Weighted average remaining contractual life, unvested shares outstanding and expected to vest	1 year 2 months 28 days	1 year 8 months 16 days	2 years 10 days
Unamortized deferred compensation cost	$ 23.1
Fair value of shares vested	18.5	$ 19.6	$ 15.4
Grant date fair value	$ 22.4	$ 32.2	$ 29.7
Stock Options [Member] \| Jones Lang Lasalle Savings Related Share Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discount on purchase price (in hundredths)	15.00%
Other Stock Compensation Programs [Abstract]
Options outstanding (in shares)	133,000	128,000
Vesting Period One [Member] \| Stock Options [Member] \| Jones Lang Lasalle Savings Related Share Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Vesting Period Two [Member] \| Stock Options [Member] \| Jones Lang Lasalle Savings Related Share Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	5 years

-- None. No documentation exists for this element. --

No definition available.

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The number of shares reserved for issuance under stock options outstanding under other stock compensation programs.

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ShareBased Compensation Arrangement By ShareBased Payment Award, Equity Instruments Other Than Options, Grants In Period, Weighted Average Grant Date Fair Value1

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The weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect during the reporting period as a result of the occurrence of a terminating event specified in the contractual agreement of the plan.

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Discount rate from fair value on purchase date that participants pay for shares.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Net number of share options (or share units) granted during the period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Number of share options (or share units) exercised during the current period.

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