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Business Combinations - Additional Information (Details) - USD ($)
$ in Thousands
Jul. 01, 2018
Sep. 30, 2019
Dec. 31, 2018
Business Acquisition [Line Items]      
Goodwill   $ 290,366 $ 290,366
Sustainable Growth Advisers, LP      
Business Acquisition [Line Items]      
Percentage of voting interests acquired 70.00%    
Purchase price $ 129,500    
Goodwill 120,213    
Intangible assets 62,000    
Expected tax deductible amount of goodwill and other intangible assets $ 127,500    
Sustainable Growth Advisers, LP | SGIA, LLC | Sustainable Growth Advisers, LP      
Business Acquisition [Line Items]      
Percentage of membership interests 100.00%