Regulatory Matters (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Company
Total Capital (to risk-weighted assets)
Total Capital (to risk-weighted assets), Actual, Capital Amount	$ 609,995	$ 393,654
Total Capital (to risk weighted assets), Actual, Ratio (as a percent)	10.80%	12.30%
Total Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 450,589	$ 255,884
Total Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	8.00%	8.00%
Tier I Capital (to risk-weighted assets)
Tier I Capital, (to risk-weighted assets), Actual, Capital Amount	$ 596,290	$ 380,950
Tier I Capital (to risk weighted assets), Actual, Ratio (as a percent)	10.60%	11.90%
Tier I Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 337,942	$ 191,913
Tier I Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	6.00%	6.00%
Common Equity Tier I Capital 1 (to risk-weighted assets)
Common Equity Tier I Capital 1, Actual, Capital Amount	$ 383,602	$ 339,369
Common Equity Tier I Capital 1 (to risk weighted assets), Ratio (as a percent)	6.80%	10.60%
Common Equity Tier I Capital 1, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 253,457	$ 143,935
Common Equity Tier I Capital 1 (to risk weighted assets, Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	4.50%	4.50%
Tier 1 Capital (to average assets)
Tier 1 Capital, Actual, Capital Amount	$ 596,290	$ 380,950
Tier 1 Capital (to average assets), Actual, Ratio (as a percent)	10.00%	10.00%
Tier 1 Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 239,096	$ 152,081
Tier 1 Capital (to average assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	4.00%	4.00%
Federal Deposit Corporation And Federal Reserve Board | Rules Effective January 1, 2015
Tier 1 Capital (to average assets)
Basel III Capital Rules related to CET1, period (in years)	4 years
Additional capital conservation buffer requirement subsequent years Ratio	2.50%
Merchants Bank
Total Capital (to risk-weighted assets)
Total Capital (to risk-weighted assets), Actual, Capital Amount	$ 613,387	$ 412,386
Total Capital (to risk weighted assets), Actual, Ratio (as a percent)	11.20%	13.30%
Total Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 439,405	$ 248,290
Total Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	8.00%	8.00%
Total capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 549,257	$ 310,363
Total Capital (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	10.00%	10.00%
Tier I Capital (to risk-weighted assets)
Tier I Capital, (to risk-weighted assets), Actual, Capital Amount	$ 600,164	$ 399,815
Tier I Capital (to risk weighted assets), Actual, Ratio (as a percent)	10.90%	12.90%
Tier I Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 329,554	$ 186,218
Tier I Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	6.00%	6.00%
Tier I Capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 439,405	$ 248,290
Tier I Capital (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	8.00%	8.00%
Common Equity Tier I Capital 1 (to risk-weighted assets)
Common Equity Tier I Capital 1, Actual, Capital Amount	$ 600,164	$ 399,815
Common Equity Tier I Capital 1 (to risk weighted assets), Ratio (as a percent)	10.90%	12.90%
Common Equity Tier I Capital 1, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 247,166	$ 139,663
Common Equity Tier I Capital 1 (to risk weighted assets, Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	4.50%	4.50%
Common Equity Tier I Capital 1, Minimum Amount To Be Well Capitalized, Capital Amount	$ 357,017	$ 201,736
Common Equity Tier I Capital 1 (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	6.50%	6.50%
Tier 1 Capital (to average assets)
Tier 1 Capital, Actual, Capital Amount	$ 600,164	$ 399,815
Tier 1 Capital (to average assets), Actual, Ratio (as a percent)	10.30%	11.00%
Tier 1 Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 232,749	$ 145,723
Tier 1 Capital (to average assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	4.00%	4.00%
Tier 1 Capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 290,937	$ 182,154
Tier 1 Capital (to average assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	5.00%	5.00%
FMBI
Total Capital (to risk-weighted assets)
Total Capital (to risk-weighted assets), Actual, Capital Amount	$ 20,154	$ 17,537
Total Capital (to risk weighted assets), Actual, Ratio (as a percent)	14.20%	18.60%
Total Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 11,315	$ 7,559
Total Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	8.00%	8.00%
Total capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 14,144	$ 9,448
Total Capital (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	10.00%	10.00%
Tier I Capital (to risk-weighted assets)
Tier I Capital, (to risk-weighted assets), Actual, Capital Amount	$ 19,672	$ 17,404
Tier I Capital (to risk weighted assets), Actual, Ratio (as a percent)	13.90%	18.40%
Tier I Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 8,486	$ 5,669
Tier I Capital (to risk weighted assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	6.00%	6.00%
Tier I Capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 11,315	$ 7,559
Tier I Capital (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	8.00%	8.00%
Common Equity Tier I Capital 1 (to risk-weighted assets)
Common Equity Tier I Capital 1, Actual, Capital Amount	$ 19,672	$ 17,404
Common Equity Tier I Capital 1 (to risk weighted assets), Ratio (as a percent)	13.90%	18.40%
Common Equity Tier I Capital 1, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 6,365	$ 4,252
Common Equity Tier I Capital 1 (to risk weighted assets, Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	4.50%	4.50%
Common Equity Tier I Capital 1, Minimum Amount To Be Well Capitalized, Capital Amount	$ 9,194	$ 6,141
Common Equity Tier I Capital 1 (to risk weighted assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	6.50%	6.50%
Tier 1 Capital (to average assets)
Tier 1 Capital, Actual, Capital Amount	$ 19,672	$ 17,404
Tier 1 Capital (to average assets), Actual, Ratio (as a percent)	11.50%	10.80%
Tier 1 Capital, Minimum Amount Required for Adequately Capitalized, Capital Amount	$ 6,845	$ 6,453
Tier 1 Capital (to average assets), Minimum Amount Required for Adequately Capitalized, Ratio (as a percent)	4.00%	4.00%
Tier 1 Capital, Minimum Amount To Be Well Capitalized, Capital Amount	$ 8,556	$ 8,066
Tier 1 Capital (to average assets), Minimum Amount To Be Well Capitalized, Ratio (as a percent)	5.00%	5.00%