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Employee Benefit Plans (FY) (Details Textual) - USD ($) |  | 12 Months Ended
--- | --- | ---
Aug. 04, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |  |
Deferred compensation arrangements, overall, description |  | After one year of service the Company matches 100 percent of employee contributions up to 3 percent of compensation and 50 percent of employee contributions on the next 2 percent of compensation. |
Share-based compensation expense |  | $ 97,966 | $ 132,635
Options vested in period, fair value |  | 313,977 | 95,658
Deferred tax expense from stock options exercised |  | 1,331,689 | 252,931
Options, exercises in period, intrinsic value |  | 5,468,780 | $ 660,476
Options, outstanding, intrinsic value |  | 3,261,940 |
Options, exercisable, intrinsic value |  | 3,105,964 |
Proceeds from stock options exercised |  | $ 4,885,646 |
Number of shares granted |  | 0 | 0
Weighted average grant-date fair value, granted (in dollars per share) |  | $ 0 |
Director [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Direct stock grant expense |  | $ 31,791 | $ 0
Deferred Salary Reduction Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized |  | 427,975 | 403,309
Stock Options [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Unrecognized compensation costs |  | $ 258,000 |
Unrecognized compensation costs, period for recognition |  | 11 months 16 days |
Stock Appreciation Rights (SARs) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Share-based compensation expense |  | $ 21,829 | 0
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 |
Stock Incentive Plan Two Thousand Fifteen [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Exercise price, not less than (as a percent) |  | 110.00% |
Number Outstanding (in shares) |  | 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% |
Stock Incentive Plan Two Thousand Fifteen [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 Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ 24.58 |  |
Share-based compensation expense |  | $ 56,330 | $ 0
Number of shares granted | 27,500 |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options |  | $ 619,620 |
401 (k) Matching Range One [Member] | Deferred Salary Reduction Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Request service period |  | 1 year |
Employer matching contribution, percent of match |  | 100.00% |
Employer matching contribution, percent of employees gross pay |  | 3.00% |
401 (k) Matching Range Two [Member] | Deferred Salary Reduction Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer matching contribution, percent of match |  | 50.00% |
Employer matching contribution, percent of employees gross pay |  | 2.00% |

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