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Summary of Significant Accounting Principles and Practices - Additional Information (Details)
£ in Millions, $ in Millions
12 Months Ended
Dec. 31, 2019
GBP (£)
Jan. 01, 2020
USD ($)
Dec. 31, 2019
USD ($)
Jan. 01, 2019
USD ($)
Dec. 31, 2018
GBP (£)
Dec. 31, 2018
USD ($)
Jan. 01, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Cash and Cash Equivalents                  
Cash and cash equivalents and short-term investments     $ 928.0     $ 828.0      
Restricted cash and investments, current     110.0     91.0      
Operating funds in U.K. £ 42.7   55.5   £ 42.7 53.9      
Fiduciary Assets and Liabilities                  
Premium trust balances     5,200.0     3,900.0      
Allowance for Doubtful Accounts                  
Allowance for doubtful accounts     70.0     $ 62.0   $ 59.0 $ 56.0
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]                  
Operating lease right-of-use assets     929.0            
Present value of lease liabilities     $ 1,130.0            
Adoption of new accounting guidance             $ 492.0    
Buildings                  
Fixed Assets                  
Fixed assets, original life, weighted-average 35 years                
Automobiles                  
Fixed Assets                  
Fixed assets, original life, weighted-average 6 years                
Minimum | Software                  
Fixed Assets                  
Fixed assets, original life, weighted-average 4 years                
Minimum | Furniture, fixtures and equipment                  
Fixed Assets                  
Fixed assets, original life, weighted-average 4 years                
Minimum | Computer equipment                  
Fixed Assets                  
Fixed assets, original life, weighted-average 4 years                
Maximum | Software                  
Fixed Assets                  
Fixed assets, original life, weighted-average 7 years                
Maximum | Leasehold improvements                  
Fixed Assets                  
Fixed assets, original life, weighted-average 10 years                
Maximum | Furniture, fixtures and equipment                  
Fixed Assets                  
Fixed assets, original life, weighted-average 10 years                
Maximum | Computer equipment                  
Fixed Assets                  
Fixed assets, original life, weighted-average 6 years                
Accounting Standards Update 2016-02                  
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]                  
Operating lease right-of-use assets       $ 1,100.0          
Present value of lease liabilities       $ 1,300.0          
Accounting Standards Update 2014-09                  
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]                  
Adoption of new accounting guidance             $ 507.0    
Accounting Standards Update 2016-13 | Subsequent Event                  
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]                  
Adoption of new accounting guidance   $ 10.0              
Tradenames | Minimum                  
Goodwill and Intangible Assets                  
Useful life of finite-lived intangible assets 1 year                
Tradenames | Maximum                  
Goodwill and Intangible Assets                  
Useful life of finite-lived intangible assets 3 years                
Customer related and contract based | Minimum                  
Goodwill and Intangible Assets                  
Useful life of finite-lived intangible assets 7 years                
Customer related and contract based | Maximum                  
Goodwill and Intangible Assets                  
Useful life of finite-lived intangible assets 20 years                
Technology | Minimum                  
Goodwill and Intangible Assets                  
Useful life of finite-lived intangible assets 5 years                
Technology | Maximum                  
Goodwill and Intangible Assets                  
Useful life of finite-lived intangible assets 7 years