Employee Benefit Plans - Narrative (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 04, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Share-based compensation expense		$ 23,718	$ 33,000	$ 74,248	$ 99,635
Options vested in period, fair value				348,124	84,010
Deferred tax expense from stock options exercised				0	0
Options, exercises in period, intrinsic value				5,451,319
Options, outstanding, intrinsic value		2,390,463		2,390,463
Options, exercisable, intrinsic value		2,179,133		2,179,133
Proceeds from stock options exercised				$ 4,873,757
Number of shares granted				0		0
Deferred Salary Reduction Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost recognized		89,463	$ 110,107	$ 298,309	300,530
Director [Member]
Defined Benefit Plan Disclosure [Line Items]
Direct stock grant expense				31,791	0
Stock Options [Member]
Defined Benefit Plan Disclosure [Line Items]
Unrecognized compensation costs		$ 282,000		$ 282,000
Unrecognized compensation costs, period for recognition				1 year
Restricted Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of shares granted				27,500
Legacy Cornerstone Bancshares, Inc. Long-Term Incentive Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Exercise price, not less than (as a percent)				100.00%
Legacy Smart Financial Inc 2010 Incentive Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Exercise price, not less than (as a percent)				100.00%
Number of shares authorized		525,000		525,000
2015 Stock Incentive Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Exercise price, not less than (as a percent)				110.00%
Number outstanding (in shares)		2,000,000		2,000,000
Term of options, no more than				10 years
Voting power held by participant (as a percent)				10.00%
Term of option to participants with more than ten percent voting power				5 years
Percentage of incentive stock options vesting on second anniversary				30.00%
Percentage of incentive stock options vesting on third anniversary				30.00%
Percentage of incentive stock options vesting on fourth anniversary				40.00%
Percentage of nonqualified stock options vesting on first anniversary				50.00%
Percentage of nonqualified stock options vesting on second anniversary				50.00%
2015 Stock Incentive Plan [Member] | Restricted Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Term of option to participants with more than ten percent voting power				5 years
Share-based compensation expense		$ 22,532		$ 22,532	$ 0
Number of shares granted	27,500
Fair value per share (in dollars per share)	$ 24.58
Unrecognized compensation costs for non-vested restricted stock		$ 653,418		$ 653,418
401 (k) Matching Range One [Member] | Deferred Salary Reduction Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Requisite service period				1 year
Employer matching contribution, percent of employees gross pay				3.00%
Employer matching contribution, percent of match				100.00%
401 (k) Matching Range Two [Member] | Deferred Salary Reduction Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer matching contribution, percent of employees gross pay				2.00%
Employer matching contribution, percent of match				50.00%