Income Taxes - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) state	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Feb. 10, 2017 USD ($)
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	24.70%	38.00%	35.00%	35.00%
Income Tax Examination [Line Items]
Income Tax Reconciliation Enacted Tax Rate Change	$ 12,100,000
Tax credits and other tax benefits from low-income housing tax credit			$ 1,100,000	$ 1,100,000
Amount recognized as a component of income tax expense related low-income housing tax credit	300,000	$ 200,000	200,000
Investments related to low-income housing tax credits	1,300,000	1,400,000
Impairment losses recognized from forfeiture or ineligibility	0		0	0
Future capital commitments related to low-income housing tax credit investments	4,800,000
Deferred Tax Assets, Net	$ 22,502,000	33,751,000
Number of states the company files income tax returns in | state	9
Unrecognized tax benefits	$ 1,244,000	1,180,000	1,359,000	$ 1,884,000
Unrecognized tax benefits that would impact effective tax rate	800,000	800,000	900,000
Reduction in unrecognized tax benefit that is reasonably possible due to a lapse of statute of limitations	$ 300,000
Duration of unrecognized tax benefits	12 months
State and Local Jurisdiction [Member]
Income Tax Examination [Line Items]
State valuation allowance			$ 0
Other Assets
Income Tax Examination [Line Items]
Deferred Tax Assets, Net	$ 22,500,000	$ 33,800,000
As Recorded by EFSC [Member]
Income Tax Examination [Line Items]
Deferred Tax Assets, State Taxes	$ 8,600,000				$ 8,625,000