Acquisitions & Divestitures - Summary of Balance Sheet Amounts Relative to Branches Sold (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)	Feb. 10, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash, Cash Equivalents, and Federal Funds Sold	$ 153,323,000				$ 198,802,000				$ 153,323,000	$ 198,802,000
Loans and Leases Receivable, Net Amount	4,054,473,000				3,114,752,000				4,054,473,000	3,114,752,000
Fixed assets, net	32,618,000				14,910,000				32,618,000	14,910,000
Accrued interest receivable	14,069,000				11,117,000				14,069,000	11,117,000
Goodwill	117,345,000				30,334,000				117,345,000	30,334,000	$ 30,334,000
Intangible assets, net	11,056,000				2,151,000				11,056,000	2,151,000
Deposits	4,156,414,000				3,233,361,000				4,156,414,000	3,233,361,000
Other borrowings	253,674,000				276,980,000				253,674,000	276,980,000
Investments trust preferred securities	2,100,000								2,100,000
Accrued interest payable	1,730,000				1,105,000				1,730,000	1,105,000
Other liabilities	37,986,000				77,244,000				37,986,000	77,244,000
Total cash paid to JCB shareholders and holders of JCB stock options												$ 29,300,000
Net income	7,529,000	$ 16,315,000	$ 11,955,000	$ 12,391,000	$ 13,628,000	$ 11,832,000	$ 12,352,000	$ 11,025,000	48,190,000	48,837,000	$ 38,450,000
Transaction value including JCB's common stock and stock options												171,000,000
Business Acquisition, Goodwill, Expected Tax Deductible Amount												87,000,000
Pro Forma [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenues									213,910,000	199,033,000
Net income									$ 47,227,000	$ 56,994,000
Diluted Earnings Per Share Pro Forma | $ / shares									$ 2.03	$ 2.42
As Recorded by JCB [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash, Cash Equivalents, and Federal Funds Sold												33,739,000
Interest-bearing Deposit Liabilities												1,715,000
Secured borrowings												148,670,000
Loans and Leases Receivable, Net Amount												685,905,000
Other Real Estate												6,762,000
Other Investments												2,695,000
Fixed assets, net												21,780,000
Accrued interest receivable												2,794,000
Goodwill												7,806,000
Intangible assets, net												25,000
Deferred Tax Assets, State Taxes												4,634,000
Other assets												19,107,000
Total assets acquired												935,632,000
Deposits												764,539,000
Other borrowings												55,430,000
Investments trust preferred securities												12,887,000
Accrued interest payable												653,000
Other liabilities												5,006,000
Total Liabilities Assumed												838,515,000
Net assets acquired (assumed)												97,117,000
Adjustments associated with acquisition [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash, Cash Equivalents, and Federal Funds Sold												0
Interest-bearing Deposit Liabilities												0
Secured borrowings												0
Loans and Leases Receivable, Net Amount												(11,094,000)
Other Real Estate												(5,082,000)
Other Investments												0
Fixed assets, net												(3,325,000)
Accrued interest receivable												0
Goodwill												(7,806,000)
Intangible assets, net												11,489,000
Deferred Tax Assets, State Taxes												3,991,000
Other assets												(296,000)
Total assets acquired												(12,123,000)
Deposits												629,000
Other borrowings												681,000
Investments trust preferred securities												(382,000)
Accrued interest payable												0
Other liabilities												65,000
Total Liabilities Assumed												993,000
Net assets acquired (assumed)												(13,116,000)
As Recorded by EFSC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash, Cash Equivalents, and Federal Funds Sold												33,739,000
Interest-bearing Deposit Liabilities												1,715,000
Secured borrowings												148,670,000
Loans and Leases Receivable, Net Amount												674,811,000
Other Real Estate												1,680,000
Other Investments												2,695,000
Fixed assets, net												18,455,000
Accrued interest receivable												2,794,000
Goodwill												0
Intangible assets, net												11,514,000
Deferred Tax Assets, State Taxes	$ 8,600,000								$ 8,600,000			8,625,000
Other assets												18,811,000
Total assets acquired												923,509,000
Deposits												765,168,000
Other borrowings												56,111,000
Investments trust preferred securities												12,505,000
Accrued interest payable												653,000
Other liabilities												5,071,000
Total Liabilities Assumed												839,508,000
Net assets acquired (assumed)												84,001,000
Total cash paid to JCB shareholders and holders of JCB stock options												$ 29,283,000
Common shares issued in relation to acquisition												141,729,000
Transaction value including JCB's common stock and stock options												$ 171,000,000
Business Acquisition, Goodwill, Expected Tax Deductible Amount												$ 87,011,000.00