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Equity Method Investments in Affiliates - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Schedule of Equity Method Investments [Line Items]    
Amortization expense during the period $ 23.2 $ 21.9
Acquired Client Relationships    
Schedule of Equity Method Investments [Line Items]    
Finite-lived intangible, future amortization expense in 2018 90.0  
Finite-lived intangible, future amortization expense in 2019 90.0  
Finite-lived intangible, future amortization expense in 2020 65.0  
Finite-lived intangible, future amortization expense in 2021 40.0  
Finite-lived intangible, future amortization expense in 2022 40.0  
Equity Method Investee    
Schedule of Equity Method Investments [Line Items]    
Revenue from equity method investments 310.7 265.6
Net income from equity method investments 169.6 148.6
Equity Method Investee | Acquired Client Relationships    
Schedule of Equity Method Investments [Line Items]    
Amortization expense during the period 30.0 $ 21.7
Finite-lived intangible, future amortization expense in 2018 100.0  
Finite-lived intangible, future amortization expense in 2019 90.0  
Finite-lived intangible, future amortization expense in 2020 90.0  
Finite-lived intangible, future amortization expense in 2021 90.0  
Finite-lived intangible, future amortization expense in 2022 $ 90.0