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Note 8 - Deposits (Details) - Summary of Deposits (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Summary of Deposits [Abstract]    
Noninterest-bearing demand (in Dollars) $ 558,738,000us-gaap_NoninterestBearingDomesticDepositDemand $ 224,580,000us-gaap_NoninterestBearingDomesticDepositDemand
Noninterest-bearing demand 24.50%mbwm_PercentageOfNonInterestBearingDomesticDepositsToDepositsDemand 20.10%mbwm_PercentageOfNonInterestBearingDomesticDepositsToDepositsDemand
Noninterest-bearing demand 148.80%mbwm_PercentageOfIncreaseDecreaseInNonInterestBearingDemandDeposits  
Interest-bearing checking (in Dollars) 413,382,000us-gaap_InterestBearingDomesticDepositChecking 197,388,000us-gaap_InterestBearingDomesticDepositChecking
Interest-bearing checking 18.20%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsChecking 17.60%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsChecking
Interest-bearing checking 109.40%mbwm_PercentageOfIncreaseDecreaseOfInterestBearingCheckingDeposits  
Money market (in Dollars) 235,587,000us-gaap_InterestBearingDomesticDepositMoneyMarket 133,369,000us-gaap_InterestBearingDomesticDepositMoneyMarket
Money market 10.30%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsMoneyMarket 11.90%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsMoneyMarket
Money market 76.60%mbwm_PercentageOfIncreaseDecreaseInMoneyMarketDeposits  
Savings (in Dollars) 330,459,000us-gaap_InterestBearingDomesticDepositSavings 52,606,000us-gaap_InterestBearingDomesticDepositSavings
Savings 14.50%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsSavings 4.70%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsSavings
Savings 528.20%mbwm_PercentageOfIncreaseOrDecreaseOfSavingsDeposits  
Time, under $100,000 (in Dollars) 181,026,000us-gaap_TimeDepositsLessThan100000 43,251,000us-gaap_TimeDepositsLessThan100000
Time, under $100,000 8.00%mbwm_PercentageOfDomesticTimeDepositsUnder100000 3.90%mbwm_PercentageOfDomesticTimeDepositsUnder100000
Time, under $100,000 318.50%mbwm_PercentageOfIncreaseDecreaseInDomesticTimeDepositsUnder100000  
Time, $100,000 and over (in Dollars) 382,120,000us-gaap_TimeDeposits100000OrMoreDomestic 254,600,000us-gaap_TimeDeposits100000OrMoreDomestic
Time, $100,000 and over 16.80%mbwm_PercentageOfDomesticTimeDeposits100000AndOver 22.80%mbwm_PercentageOfDomesticTimeDeposits100000AndOver
Time, $100,000 and over 50.10%mbwm_PercentageOfIncreaseDecreaseInDomesticTimeDeposits100000AndOver  
(in Dollars) 2,101,312,000mbwm_AggregateLocalDepositLiabilities 905,794,000mbwm_AggregateLocalDepositLiabilities
92.30%mbwm_AggregatePercentageOfLocalDeposits 81.00%mbwm_AggregatePercentageOfLocalDeposits
132.00%mbwm_AggregatePercentageIncreaseDecreaseInLocalDeposits  
Out-of-area time, under $100,000 (in Dollars) 2,422,000mbwm_OutOfAreaTimeDepositsUnder100000 4,078,000mbwm_OutOfAreaTimeDepositsUnder100000
Out-of-area time, under $100,000 0.10%mbwm_PercentageOfOutOfAreaTimeDepositUnder100000 0.40%mbwm_PercentageOfOutOfAreaTimeDepositUnder100000
Out-of-area time, under $100,000 (40.60%)mbwm_PercentageOfIncreaseDecreaseInOutOfAreaTimeUnder100000  
Out-of-area time, $100,000 and over (in Dollars) 173,181,000mbwm_OutOfAreaTimeDeposits100000AndOver 209,039,000mbwm_OutOfAreaTimeDeposits100000AndOver
Out-of-area time, $100,000 and over 7.60%mbwm_PercentageOfOutOfAreaTimeDeposits100000AndOver 18.60%mbwm_PercentageOfOutOfAreaTimeDeposits100000AndOver
Out-of-area time, $100,000 and over (17.20%)mbwm_PercentageOfIncreaseDecreaseInOutOfAreaTime100000AndOver  
(in Dollars) 175,603,000mbwm_AggregateOutOfAreaTimeDeposits 213,117,000mbwm_AggregateOutOfAreaTimeDeposits
7.70%mbwm_AggregatePercentageOfOutOfAreaDeposits 19.00%mbwm_AggregatePercentageOfOutOfAreaDeposits
(17.60%)mbwm_AggregatePercentageIncreaseDecreaseInOutOfAreaDeposits  
Total deposits (in Dollars) $ 2,276,915,000us-gaap_Deposits $ 1,118,911,000us-gaap_Deposits
Total deposits 100.00%mbwm_AggregatePercentageOfDeposits 100.00%mbwm_AggregatePercentageOfDeposits
Total deposits 103.50%mbwm_AggregateIncreaseDecreaseInPercentageOfDeposits