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Acquisitions & Divestitures - Narrative (Details)
 9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Feb. 10, 2017
USD ($)
$ / shares
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Number of operating JCB branches     13
Total cash paid to JCB shareholders and holders of JCB stock options     $ 29,300,000
Cash consideration per share to JCB shareholders | $ / shares     $ 85.39
Business Acquisition, Goodwill, Expected Tax Deductible Amount     $ 87,000,000
Common stock offered per share     2.75
Total shares awarded to JCB shareholders     3,300,000
EFSC Closing Stock Price     $ 42.95
Transaction value including JCB's common stock and stock options     $ 171,000,000
Business Combination, Acquisition Related Costs $ 6,500,000 $ 1,400,000  
Fair Value Discount Recorded to Portfolio Loans 24,700,000    
Fair Value Discount Recorded to Fixed Assets 1,100,000    
Assets $ 200,000