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Note 3 - Securities (Details) - Debt Securities by Maturity (USD $)	Mar. 31, 2015
Note 3 - Securities (Details) - Debt Securities by Maturity [Line Items]
Due in 2015	0.71%mbwm_WeightedAverageYieldOfSecuritiesPortfolioDueInOneYearOrLess
Due in 2015	$ 23,762,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost
Due in 2015	23,781,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
Due in 2016 through 2020	1.44%mbwm_WeightedAverageYieldOfSecuritiesPortfolioDueFromOneToFiveYears
Due in 2016 through 2020	145,330,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost
Due in 2016 through 2020	145,610,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
Due in 2021 through 2025	3.49%mbwm_WeightedAverageYieldOfSecuritiesPortfolioDueFromFiveToTenYears
Due in 2021 through 2025	67,554,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost
Due in 2021 through 2025	68,163,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
Due in 2026 and beyond	3.58%mbwm_WeightedAverageYieldOfSecuritiesPortfolioDueAfterTenYears
Due in 2026 and beyond	85,612,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost
Due in 2026 and beyond	86,909,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
	2.19%mbwm_WeightedAverageYieldOfSecuritiesPortfolio
	410,485,000us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
	413,693,000us-gaap_AvailableForSaleSecuritiesDebtSecurities
Collateralized Mortgage Backed Securities [Member]
Note 3 - Securities (Details) - Debt Securities by Maturity [Line Items]
No single maturity, weighted average yield	1.78%mbwm_WeightedAverageYieldOfSecuritiesPortfolioNoSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
No single maturity, amortized cost	86,295,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
No single maturity, fair value	87,282,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Other Debt Obligations [Member]
Note 3 - Securities (Details) - Debt Securities by Maturity [Line Items]
No single maturity, weighted average yield	2.50%mbwm_WeightedAverageYieldOfSecuritiesPortfolioNoSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember
No single maturity, amortized cost	1,932,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember
No single maturity, fair value	$ 1,948,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember

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The weighted average yield of a securities portfolio with due dates after 1 year and before 5 years.

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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-Topic 210

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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