Acquisitions & Divestitures - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Feb. 10, 2017 USD ($) $ / shares	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of operating JCB branches			13
Total shares awarded to JCB shareholders		0	3,299,864	0
Sale of Stock, Consideration Received Per Transaction | $ / shares			$ 85.39
EFSC common stock offered per share of JCB common stock			2.75
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