Acquisitions & Divestitures - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 shares $ / shares	Feb. 10, 2017 USD ($) shares branch $ / shares
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration per share to TCC shareholders | $ / shares			$ 1.84
Number of operating JCB branches | branch				13
Total shares awarded to JCB shareholders	3,299,864	0	0	3,000,000
Sale of Stock, Consideration Received Per Transaction | $ / shares				$ 85
EFSC common stock offered per share of JCB common stock | shares			0	3
Total cash paid to JCB shareholders and holders of JCB stock options				$ 29,300,000
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
Business Acquisition, Goodwill, Expected Tax Deductible Amount				$ 87,000,000
Fair Value Discount Recorded to Portfolio Loans	$ 24,700,000