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Acquisitions & Divestitures - Narrative (Details)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Feb. 10, 2017
USD ($)
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       85.39
Business Acquisition, Goodwill, Expected Tax Deductible Amount       $ 85,900,000
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   $ 6,100,000 $ 1,400,000  
Fair Value Discount Recorded to Portfolio Loans $ 24,700,000 24,700,000    
Loan fair value discount adjustment 500,000      
Core Deposit Intangible Asset Adjustment 1,500,000      
Assets $ 2,900,000 $ 2,900,000