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Business Combinations - Pro Forma Information (Details) - Sustainable Growth Advisers, LP - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Business Acquisition [Line Items]    
Total Revenues $ 569,465 $ 454,156
Net Income (Loss) Attributable to Common Stockholders $ 69,341 $ 26,175