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Summary of Significant Accounting Principles and Practices (Details) (USD $)
12 Months Ended
Dec. 31, 2011
Y
Dec. 31, 2010
Dec. 31, 2009
Cash and Cash Equivalents      
Cash and cash equivalents, restricted $ 191,000,000 $ 60,000,000  
Operating funds in U.K. 120,000,000    
Fiduciary Assets and Liabilities      
Premium trust balances 4,200,000,000 3,500,000,000  
Allowance for Doubtful Accounts      
Allowance for doubtful accounts receivable 104,000,000 102,000,000  
Goodwill and Intangible Assets      
Minimum useful life of finite-lived intangible assets (in years) 1    
Maximum useful life of finite-lived intangible assets (in years) 13    
Weighted-average original life of finite-lived intangible assets (in years) 10    
Foreign Currency      
Effect of foreign exchange gains (losses) on the consolidated statements of income 10,000,000 (18,000,000) (26,000,000)
Derivative losses $ 20,000,000 $ 11,000,000 $ 15,000,000
Buildings
     
Fixed Assets      
Fixed assets, original life, weighted-average (in years) 35    
Automobiles
     
Fixed Assets      
Fixed assets, original life, weighted-average (in years) 6    
Minimum
     
Revenue Recognition      
Term of outsourcing contracts for benefit services (in years) 3    
Term of outsourcing contracts for HR BPO services (in years) 5    
Short-term Investments      
Short-term investment, maturity period (in months/year) 3    
Consolidation of Variable Interest Entities      
Ownership percentage, criteria for consolidation 50.00%    
Minimum | Software
     
Fixed Assets      
Fixed assets, useful life, minimum (in years) 3    
Minimum | Furniture, fixtures and equipment
     
Fixed Assets      
Fixed assets, useful life, minimum (in years) 4    
Minimum | Computer equipment
     
Fixed Assets      
Fixed assets, useful life, minimum (in years) 4    
Maximum
     
Revenue Recognition      
Term of outsourcing contracts for benefit services (in years) 5    
Term of outsourcing contracts for HR BPO services (in years) 10    
Cash and Cash Equivalents      
Cash and cash equivalents, maturity period (in months) 3    
Short-term Investments      
Short-term investment, maturity period (in months/year) 1    
Maximum | Software
     
Fixed Assets      
Fixed assets, useful life, maximum (in years) 7    
Maximum | Furniture, fixtures and equipment
     
Fixed Assets      
Fixed assets, useful life, maximum (in years) 10    
Maximum | Computer equipment
     
Fixed Assets      
Fixed assets, useful life, maximum (in years) 6