Acquisitions & Divestitures - Narrative (Details)	12 Months Ended
	Dec. 31, 2016 USD ($)	Feb. 10, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets - JCB	$ 937,000,000
Loans - JCB	699,000,000
Deposits - JCB	$ 767,000,000
Number of operating JCB branches	13
Sale of Stock, Consideration Received Per Transaction		$ 85.39
EFSC common stock offered per share of JCB common stock		2.75
Total shares awarded to JCB shareholders		3,300,000
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	$ 1,400,000