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Borrowings - Senior notes and Subordinate Notes (Details) - USD ($)
$ in Millions
May 25, 2017
Sep. 30, 2019
Dec. 31, 2018
Long-term debt      
Unamortized debt issuance cost   $ 5.4 $ 5.6
Senior notes      
Long-term debt      
Issuance of debt $ 40.0    
Interest rate (as a percent) 3.75%    
Unamortized debt issuance cost   0.4 0.5
Subordinated debt      
Long-term debt      
Issuance of debt $ 60.0    
Interest rate (as a percent) 4.75%    
Term of debt instrument 5 years    
Unamortized debt issuance cost   $ 0.8 $ 0.9
Three-month LIBOR | Subordinated debt      
Long-term debt      
Floating interest rate margin (as a percent) 2.919%