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Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based Payment Arrangement, Expense $ 74.9 $ 75.3 $ 80.1  
Other Stock Compensation Programs [Abstract]        
Amortization of Other Deferred Charges (Reversal) $ 13.5      
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) 281,000      
Performance Shares [Member]        
Restricted stock unit activity [Roll Forward]        
Unvested at beginning of period (in shares) 567,000.0 646,000.0 531,500  
Granted (in shares) 185,200 116,500 218,900  
Vested (in shares) (257,200) (175,100) (79,000.0)  
Forfeited (in shares) (36,900) (20,400) (25,400)  
Unvested at end of period (in shares) 458,100 567,000.0 646,000.0 531,500
Share-based Payment Arrangement [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares available for grant (in shares) 910,000      
Restricted stock unit activity [Roll Forward]        
Unvested at beginning of period (in shares) 1,408,300 1,558,400 1,627,700  
Granted (in shares) 705,700 469,700 554,600  
Vested (in shares) (554,000.0) (570,800) (550,100)  
Forfeited (in shares) (111,800) (49,000.0) (73,800)  
Unvested at end of period (in shares) 1,448,200 1,408,300 1,558,400 1,627,700
Restricted stock unit activity, additional disclosures [Abstract]        
Weighted average grant date fair value, beginning of period (in dollars per share) $ 170.78 $ 152.27 $ 137.42  
Weighted average grant date fair value, granted (in dollars per share) 143.64 204.05 189.40  
Weighted average grant date fair value, vested (in dollars per share) 126.28 147.78 145.54  
Weighted average grant date fair value, forfeitures (in dollars per share) 164.42 168.92 148.97  
Weighted average grant date fair value, end of period (in dollars per share) $ 175.07 $ 170.78 $ 152.27 $ 137.42
Weighted average remaining contractual life, unvested shares outstanding and expected to vest 1 year 7 months 28 days 1 year 9 months 14 days 1 year 11 months 26 days 1 year 8 months 8 days
Unamortized deferred compensation cost $ 79.2      
Fair value of shares vested 70.0 $ 84.3 $ 80.1  
Grant date fair value 101.4 95.8 105.0  
Share-based Payment Arrangement [Member] | Performance Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based Payment Arrangement, Expense 9.0 26.8 42.4  
Share-based Payment Arrangement [Member] | Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based Payment Arrangement, Expense $ 65.9 $ 48.5 $ 37.7  
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%      
Restricted Stock Units (RSUs) [Member]        
Restricted stock unit activity [Roll Forward]        
Unvested at beginning of period (in shares) 841,300 912,400 1,096,200  
Granted (in shares) 520,500 353,200 335,700  
Vested (in shares) (296,800) (395,700) (471,100)  
Forfeited (in shares) (74,900) (28,600) (48,400)  
Unvested at end of period (in shares) 990,100 841,300 912,400 1,096,200