Business Combinations - HealthTran (Details) (USD $)
 12 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jan. 31, 2012
Dec. 31, 2011
Business Acquisition [Line Items]          
Amortization of acquired intangible assets $ 213,220,000 $ 203,192,000 $ 130,116,000    
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]          
Goodwill 4,724,639,000 4,720,275,000 4,478,038,000    
Minimum          
Business Acquisition [Line Items]          
Finite-lived intangibles, estimated useful life 1 year        
Maximum          
Business Acquisition [Line Items]          
Finite-lived intangibles, estimated useful life 15 years        
Customer Relationships | Minimum          
Business Acquisition [Line Items]          
Finite-lived intangibles, estimated useful life 3 years        
Customer Relationships | Maximum          
Business Acquisition [Line Items]          
Finite-lived intangibles, estimated useful life 10 years        
HealthTran          
Business Acquisition [Line Items]          
Cash paid to shareholders       250,000,000  
Acquisition costs included in selling, general and administrative expenses         900,000
Amortization of acquired intangible assets 14,600,000 12,600,000      
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]          
Intangible assets       74,530,000  
HealthTran | Trade Mark Trade Name          
Business Acquisition [Line Items]          
Finite-lived intangibles, estimated useful life       6 months  
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]          
Intangible assets       1,750,000  
HealthTran | Customer Relationships          
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]          
Intangible assets       69,800,000  
HealthTran | Customer Relationships | Minimum          
Business Acquisition [Line Items]          
Finite-lived intangibles, estimated useful life       4 years  
HealthTran | Customer Relationships | Maximum          
Business Acquisition [Line Items]          
Finite-lived intangibles, estimated useful life       9 years  
HealthTran | Non-compete agreements          
Business Acquisition [Line Items]          
Finite-lived intangibles, estimated useful life       5 years  
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]          
Intangible assets       2,600,000  
HealthTran | Licensing Agreements          
Business Acquisition [Line Items]          
Finite-lived intangibles, estimated useful life       3 years  
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]          
Intangible assets       380,000  
HealthTran | Initial Amounts Recognized at Acquisition Date          
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]          
Current assets       30,654,000  
Property and equipment       2,787,000  
Goodwill       173,642,000  
Intangible assets       77,130,000  
Total assets acquired       284,213,000  
Current liabilities       36,784,000  
Total liabilities assumed       36,784,000  
Net assets acquired       247,429,000  
HealthTran | Measurement Period Adjustments          
Business Acquisition [Line Items]          
Adjustment paid to former owners       1,000,000  
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]          
Current assets       245,000  
Property and equipment       0  
Goodwill       833,000  
Intangible assets       (2,600,000)  
Total assets acquired       (1,522,000)  
Current liabilities       (496,000)  
Total liabilities assumed       (496,000)  
Net assets acquired       (1,026,000)  
HealthTran | Final Amounts Recognized at Acquisition Date          
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]          
Current assets       30,899,000  
Property and equipment       2,787,000  
Goodwill       174,475,000  
Intangible assets       74,530,000  
Total assets acquired       282,691,000  
Current liabilities       36,288,000  
Total liabilities assumed       36,288,000  
Net assets acquired       $ 246,403,000