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Subsequent Event (Details) - USD ($)
$ in Millions
2 Months Ended
Jan. 01, 2018
Feb. 28, 2018
Dec. 31, 2017
Subsequent Event [Line Items]      
Percentage of equity owned     100.00%
Subsequent Event | Subsidiary | S&T Evergreen Insurance, LLC      
Subsequent Event [Line Items]      
Proceeds from sale of majority interest in subsidiary $ 5.0    
Recognized gain on sale of subsidiary   $ 1.0  
Subsequent Event | S&T Evergreen Insurance, LLC      
Subsequent Event [Line Items]      
Percentage of equity owned 30.00%