Acquisitions & Divestitures - Summary of Balance Sheet Amounts Relative to Branches Sold (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) $ / shares	Jun. 30, 2016 USD ($) $ / shares	Feb. 10, 2017 USD ($)	Dec. 31, 2016 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net income as reported	$ 11,955,000	$ 12,352,000	$ 24,346,000	$ 23,377,000
Cash and cash equivalents	117,764,000		117,764,000			$ 198,802,000
Property, Plant and Equipment, Net	33,987,000		33,987,000			14,910,000
Interest Receivable	11,923,000		11,923,000			11,117,000
Goodwill	116,186,000		116,186,000			30,334,000
Other Intangible Assets	12,458,000		12,458,000			2,151,000
Deposits	3,921,251,000		3,921,251,000			3,233,361,000
Other Borrowings	217,180,000		217,180,000			276,980,000
Interest Payable	1,459,000		1,459,000			1,105,000
Other Liabilities	$ 30,981,000		$ 30,981,000			$ 77,244,000
Total cash paid to JCB shareholders and holders of JCB stock options					$ 29,300,000
Common shares issued related to acquisition	141,729,000		141,729,000			0
Transaction value including JCB's common stock and stock options					171,000,000
Business Acquisition, Goodwill, Expected Tax Deductible Amount					85,900,000
Pro Forma [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenues			$ 101,954,000	97,208,000
Net income as reported			$ 23,845,000	$ 27,721,000
Diluted Earnings Per Share Pro Forma | $ / shares			$ 1.00	$ 1.18
As Recorded by JCB [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and cash equivalents					33,739,000
Interest-bearing Deposit Liabilities					1,715,000
Securities					148,670,000
Portfolio loans, net					685,905,000
Other real estate owned					6,762,000
Other Investments					2,695,000
Property, Plant and Equipment, Net					21,780,000
Interest Receivable					2,794,000
Goodwill					7,806,000
Other Intangible Assets					25,000
Deferred Tax Assets					4,634,000
Other Assets					19,107,000
Total assets acquired					935,632,000
Deposits					764,539,000
Other Borrowings					55,430,000
Trust preferred securities					12,887,000
Interest Payable					653,000
Other Liabilities					5,006,000
Total Liabilities Assumed					838,515,000
Net assets acquired					97,117,000
Adjustments associated with acquisition [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and cash equivalents					0
Interest-bearing Deposit Liabilities					0
Securities					0
Portfolio loans, net					(11,094,000)
Other real estate owned					(5,082,000)
Other Investments					0
Property, Plant and Equipment, Net					(2,259,000)
Interest Receivable					0
Goodwill					(7,806,000)
Other Intangible Assets					11,489,000
Deferred Tax Assets					4,144,000
Other Assets					(296,000)
Total assets acquired					(10,904,000)
Deposits					629,000
Other Borrowings					681,000
Trust preferred securities					(382,000)
Interest Payable					0
Other Liabilities					125,000
Total Liabilities Assumed					1,053,000
Net assets acquired					(11,957,000)
As Recorded by EFSC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and cash equivalents					33,739,000
Interest-bearing Deposit Liabilities					1,715,000
Securities					148,670,000
Portfolio loans, net					674,811,000
Other real estate owned					1,680,000
Other Investments					2,695,000
Property, Plant and Equipment, Net					19,521,000
Interest Receivable					2,794,000
Goodwill					0
Other Intangible Assets					11,514,000
Deferred Tax Assets					8,778,000
Other Assets					18,811,000
Total assets acquired					924,728,000
Deposits					765,168,000
Other Borrowings					56,111,000
Trust preferred securities					12,505,000
Interest Payable					653,000
Other Liabilities					5,131,000
Total Liabilities Assumed					839,568,000
Net assets acquired					85,160,000
Total cash paid to JCB shareholders and holders of JCB stock options					$ 29,283,000
Common shares issued related 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					$ 85,852,000.00