Acquisitions & Divestitures - Narrative (Details)		12 Months Ended
	Mar. 08, 2019 USD ($) office $ / shares shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Mar. 07, 2019 $ / shares	Feb. 10, 2017 USD ($) shares branch $ / shares
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash, Cash Equivalents, and Federal Funds Sold		$ 167,256,000	$ 196,552,000
Number of operating JCB branches | branch						13
Total shares awarded to JCB shareholders | shares						3,300,000
Sale of Stock, Consideration Received Per Transaction | $ / shares						$ 85.39
EFSC common stock offered per share of JCB common stock | shares						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						$ 171,000,000.0
Business Combination, Acquisition Related Costs				$ 6,500,000
Interest-earning deposits greater than 90 days		3,730,000	3,185,000
Securities available-for-sale		1,135,317,000	721,369,000
Loans and Leases Receivable, Net Amount		5,271,049,000	4,306,525,000
Fixed assets, net		60,013,000	32,109,000
Intangible assets, net		26,076,000	8,553,000
Deposits		5,771,023,000	4,587,985,000
Subordinated debentures and notes		141,258,000	118,156,000
Other liabilities		66,747,000	42,267,000
Goodwill		210,344,000	117,345,000
As Recorded by Acquiree [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash, Cash Equivalents, and Federal Funds Sold	$ 13,899,000
Interest-earning deposits greater than 90 days	100,000
Securities available-for-sale	428,715,000
Loans and Leases Receivable, Net Amount	705,057,000
Other Real Estate	5,284,000
Other Investments	6,673,000
Fixed assets, net	27,586,000
Accrued interest receivable	3,997,000
Intangible assets, net	0
Deferred Tax Assets, State Taxes	10,708,000
Other assets	35,045,000
Total assets acquired	1,237,064,000
Deposits	1,081,151,000
Subordinated debentures and notes	26,806,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	6,800,000
Accrued interest payable	370,000
Other liabilities	5,842,000
Total Liabilities Assumed	1,120,969,000
Net assets acquired (assumed)	116,095,000
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-earning deposits greater than 90 days	0
Securities available-for-sale	(619,000)
Loans and Leases Receivable, Net Amount	(20,743,000)
Other Real Estate	(2,059,000)
Other Investments	0
Fixed assets, net	(300,000)
Accrued interest receivable	0
Intangible assets, net	23,066,000
Deferred Tax Assets, State Taxes	(2,386,000)
Other assets	(1,484,000)
Total assets acquired	(4,525,000)
Deposits	36,000
Subordinated debentures and notes	(3,972,000)
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	171,000
Accrued interest payable	0
Other liabilities	(827,000)
Total Liabilities Assumed	(4,592,000)
Net assets acquired (assumed)	$ 67,000
Trinity Capital Corporation [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Banking Offices | office	6
Business Acquisition, Share Price | $ / shares	$ 1.84
Business Acquisition, Equity Interest Issued or Issuable Per Acquired Share	0.1972
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares	4,000,000.0
Payments to Acquire Businesses, Gross	$ 37,275,000
Share Price | $ / shares					$ 43.07
Business Combination, Consideration Transferred	209,160,000
Business Acquisition, Transaction Costs		18,000,000.0	1,300,000
Goodwill, Acquired During Period	93,000,000.0
Cash, Cash Equivalents, and Federal Funds Sold	13,899,000
Business Acquisition, Pro Forma Revenue		296,677,000	286,076,000
Business Acquisition, Pro Forma Net Income (Loss)		$ 107,626,000	$ 85,579,000
Business Acquisition, Pro Forma Earnings Per Share, Diluted | $ / shares		$ 4.11	$ 3.14
Interest-earning deposits greater than 90 days	100,000
Securities available-for-sale	428,096,000
Loans and Leases Receivable, Net Amount	684,314,000
Other Real Estate	3,225,000
Other Investments	6,673,000
Fixed assets, net	27,286,000
Accrued interest receivable	3,997,000
Intangible assets, net	23,066,000
Deferred Tax Assets, State Taxes	8,322,000
Other assets	33,561,000
Total assets acquired	1,232,539,000
Deposits	1,081,187,000
Subordinated debentures and notes	22,834,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	6,971,000
Accrued interest payable	370,000
Other liabilities	5,015,000
Total Liabilities Assumed	1,116,377,000
Net assets acquired (assumed)	116,162,000
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable	171,885,000
Goodwill	$ 92,998,000
Core Deposits [Member] | Trinity Capital Corporation [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	10 years