Regulatory Matters (Details) $ in Thousands	Sep. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Company
Total Capital (to risk-weighted assets)
Total Capital (to risk-weighted assets), Actual, Capital Amount	$ 1,454,722
Total Capital (to risk weighted assets), Actual, Ratio (as a percent)	0.125
Total Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 932,855
Total Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.080
Tier I Capital (to risk-weighted assets)
Tier I Capital, (to risk-weighted assets), Actual, Capital Amount	$ 1,405,621
Tier I Capital (to risk weighted assets), Actual, Ratio (as a percent)	0.121
Tier I Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 699,641
Tier I Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.060
Common Equity Tier I Capital 1 (to risk-weighted assets)
Common Equity Tier I Capital 1, Actual, Capital Amount	$ 906,102
Common Equity Tier I Capital 1 (to risk weighted assets), Ratio (as a percent)	0.078
Common Equity Tier I Capital 1, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 524,731
Common Equity Tier I Capital 1 (to risk weighted assets, Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.045%
Tier 1 Capital (to average assets)
Tier 1 Capital, Actual, Capital Amount	$ 1,405,621	$ 1,138,090
Tier 1 Capital (to average assets), Actual, Ratio (as a percent)	0.123	0.104
Tier 1 Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 456,766
Tier 1 Capital (to average assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.040
Tier 1 Capital, Minimum Amount To Be Well Capitalized, Capital Amount		$ 928,731
Company | Minimum
Tier 1 Capital (to average assets)
Tier 1 Capital (to average assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)		0.085
Merchants Bank
Total Capital (to risk-weighted assets)
Total Capital (to risk-weighted assets), Actual, Capital Amount	$ 1,376,248
Total Capital (to risk weighted assets), Actual, Ratio (as a percent)	0.120
Total Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 915,426
Total Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.080
Total capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 1,144,283
Total Capital (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	0.100
Tier I Capital (to risk-weighted assets)
Tier I Capital, (to risk-weighted assets), Actual, Capital Amount	$ 1,327,929
Tier I Capital (to risk weighted assets), Actual, Ratio (as a percent)	0.116
Tier I Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 686,570
Tier I Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.060
Tier I Capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 915,426
Tier I Capital (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	0.080
Common Equity Tier I Capital 1 (to risk-weighted assets)
Common Equity Tier I Capital 1, Actual, Capital Amount	$ 1,327,929
Common Equity Tier I Capital 1 (to risk weighted assets), Ratio (as a percent)	0.116
Common Equity Tier I Capital 1, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 514,927
Common Equity Tier I Capital 1 (to risk weighted assets, Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.045%
Common Equity Tier I Capital 1, Minimum Amount To Be Well Capitalized, Capital Amount	$ 743,784
Common Equity Tier I Capital 1 (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	0.065%
Tier 1 Capital (to average assets)
Tier 1 Capital, Actual, Capital Amount	$ 1,327,929	$ 1,088,621
Tier 1 Capital (to average assets), Actual, Ratio (as a percent)	0.119	0.103
Tier 1 Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 446,683
Tier 1 Capital (to average assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.040
Tier 1 Capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 558,354	$ 901,188
Tier 1 Capital (to average assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	0.050
Merchants Bank | Minimum
Tier 1 Capital (to average assets)
Tier 1 Capital (to average assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)		0.085
FMBI
Total Capital (to risk-weighted assets)
Total Capital (to risk-weighted assets), Actual, Capital Amount	$ 33,514
Total Capital (to risk weighted assets), Actual, Ratio (as a percent)	0.115
Total Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 23,351
Total Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.080
Total capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 29,189
Total Capital (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	0.100
Tier I Capital (to risk-weighted assets)
Tier I Capital, (to risk-weighted assets), Actual, Capital Amount	$ 32,732
Tier I Capital (to risk weighted assets), Actual, Ratio (as a percent)	0.112
Tier I Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 17,513
Tier I Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.060
Tier I Capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 23,351
Tier I Capital (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	0.080
Common Equity Tier I Capital 1 (to risk-weighted assets)
Common Equity Tier I Capital 1, Actual, Capital Amount	$ 32,732
Common Equity Tier I Capital 1 (to risk weighted assets), Ratio (as a percent)	0.112
Common Equity Tier I Capital 1, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 13,135
Common Equity Tier I Capital 1 (to risk weighted assets, Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.045%
Common Equity Tier I Capital 1, Minimum Amount To Be Well Capitalized, Capital Amount	$ 18,973
Common Equity Tier I Capital 1 (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	0.065%
Tier 1 Capital (to average assets)
Tier 1 Capital, Actual, Capital Amount	$ 32,732	$ 28,958
Tier 1 Capital (to average assets), Actual, Ratio (as a percent)	0.105	0.097
Tier 1 Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 12,499
Tier 1 Capital (to average assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	0.040
Tier 1 Capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 15,624	$ 25,499
Tier 1 Capital (to average assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	0.050
FMBI | Minimum
Tier 1 Capital (to average assets)
Tier 1 Capital (to average assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)		0.085