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Note 6 - Deposits (Details) - Summary of Deposits (USD $)
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Summary of Deposits [Abstract]    
Noninterest-bearing checking (in Dollars) $ 568,843,000us-gaap_NoninterestBearingDomesticDepositDemand $ 558,738,000us-gaap_NoninterestBearingDomesticDepositDemand
Noninterest-bearing checking 25.00%mbwm_PercentageOfNonInterestBearingDomesticDepositsToDepositsDemand 24.50%mbwm_PercentageOfNonInterestBearingDomesticDepositsToDepositsDemand
Noninterest-bearing checking 1.80%mbwm_PercentageOfIncreaseDecreaseInNonInterestBearingDemandDeposits  
Interest-bearing checking (in Dollars) 417,098,000us-gaap_InterestBearingDomesticDepositChecking 413,382,000us-gaap_InterestBearingDomesticDepositChecking
Interest-bearing checking 18.30%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsChecking 18.20%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsChecking
Interest-bearing checking 0.90%mbwm_PercentageOfIncreaseDecreaseOfInterestBearingCheckingDeposits  
Money market (in Dollars) 250,421,000us-gaap_InterestBearingDomesticDepositMoneyMarket 235,587,000us-gaap_InterestBearingDomesticDepositMoneyMarket
Money market 11.00%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsMoneyMarket 10.30%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsMoneyMarket
Money market 6.30%mbwm_PercentageOfIncreaseDecreaseInMoneyMarketDeposits  
Savings (in Dollars) 345,459,000us-gaap_InterestBearingDomesticDepositSavings 330,459,000us-gaap_InterestBearingDomesticDepositSavings
Savings 15.20%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsSavings 14.50%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsSavings
Savings 4.50%mbwm_PercentageOfIncreaseOrDecreaseOfSavingsDeposits  
Time, under $100,000 (in Dollars) 175,433,000us-gaap_TimeDepositsLessThan100000 181,026,000us-gaap_TimeDepositsLessThan100000
Time, under $100,000 7.70%mbwm_PercentageOfDomesticTimeDepositsUnder100000 8.00%mbwm_PercentageOfDomesticTimeDepositsUnder100000
Time, under $100,000 (3.10%)mbwm_PercentageOfIncreaseDecreaseInDomesticTimeDepositsUnder100000  
Time, $100,000 and over (in Dollars) 368,887,000us-gaap_TimeDeposits100000OrMoreDomestic 382,120,000us-gaap_TimeDeposits100000OrMoreDomestic
Time, $100,000 and over 16.10%mbwm_PercentageOfDomesticTimeDeposits100000AndOver 16.80%mbwm_PercentageOfDomesticTimeDeposits100000AndOver
Time, $100,000 and over (3.50%)mbwm_PercentageOfIncreaseDecreaseInDomesticTimeDeposits100000AndOver  
(in Dollars) 2,126,141,000mbwm_AggregateLocalDepositLiabilities 2,101,312,000mbwm_AggregateLocalDepositLiabilities
93.30%mbwm_AggregatePercentageOfLocalDeposits 92.30%mbwm_AggregatePercentageOfLocalDeposits
1.20%mbwm_AggregatePercentageIncreaseDecreaseInLocalDeposits  
Out-of-area time, under $100,000 (in Dollars) 1,828,000mbwm_OutOfAreaTimeDepositsUnder100000 2,422,000mbwm_OutOfAreaTimeDepositsUnder100000
Out-of-area time, under $100,000 0.10%mbwm_PercentageOfOutOfAreaTimeDepositUnder100000 0.10%mbwm_PercentageOfOutOfAreaTimeDepositUnder100000
Out-of-area time, under $100,000 (24.50%)mbwm_PercentageOfIncreaseDecreaseInOutOfAreaTimeUnder100000  
Out-of-area time, $100,000 and over (in Dollars) 151,555,000mbwm_OutOfAreaTimeDeposits100000AndOver 173,181,000mbwm_OutOfAreaTimeDeposits100000AndOver
Out-of-area time, $100,000 and over 6.60%mbwm_PercentageOfOutOfAreaTimeDeposits100000AndOver 7.60%mbwm_PercentageOfOutOfAreaTimeDeposits100000AndOver
Out-of-area time, $100,000 and over (12.50%)mbwm_PercentageOfIncreaseDecreaseInOutOfAreaTime100000AndOver  
(in Dollars) 153,383,000mbwm_AggregateOutOfAreaTimeDeposits 175,603,000mbwm_AggregateOutOfAreaTimeDeposits
6.70%mbwm_AggregatePercentageOfOutOfAreaDeposits 7.70%mbwm_AggregatePercentageOfOutOfAreaDeposits
(12.70%)mbwm_AggregatePercentageIncreaseDecreaseInOutOfAreaDeposits  
Total deposits (in Dollars) $ 2,279,524,000us-gaap_Deposits $ 2,276,915,000us-gaap_Deposits
Total deposits 100.00%mbwm_AggregatePercentageOfDeposits 100.00%mbwm_AggregatePercentageOfDeposits
Total deposits 0.10%mbwm_AggregateIncreaseDecreaseInPercentageOfDeposits