Note 2 - Business Combination (Tables)

12 Months Ended

Dec. 31, 2014

Business Combinations [Abstract]

Schedule of Business Acquisitions by Acquisition, Equity Interest Issued or Issuable [Table Text Block]

Purchase Price:
Mercantile common shares issued for Firstbank common shares		8,087,272
Price per share, based on Mercantile closing price on May 30, 2014	$	21.43
Value of common stock issued		173,310,000
Value of replacement stock options granted		1,664,000
Total purchase price	$	174,974,000

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block]

Preliminary Statement of Net Assets Acquired at Fair Value:
Assets
Cash and cash equivalents	$	91,806,000
Securities		358,599,000
Total loans		943,662,000
Premises and equipment		24,049,000
Core deposit intangible		17,478,000
Mortgage servicing rights		7,389,000
Other assets		9,897,000
Total Assets	$	1,452,880,000
Liabilities
Deposits	$	1,229,609,000
Borrowings		87,615,000
Other liabilities		10,155,000
Total Liabilities	$	1,327,379,000
Net Identifiable Assets Acquired			$	125,501,000
Goodwill			$	49,473,000

Business Acquisition, Pro Forma Information [Table Text Block]

	2014		2013
Net interest income	$	98,607,000	$	93,862,000
Noninterest expense		81,295,000		93,263,000
Net income		22,659,000		16,068,000
Net income per diluted share		1.33		0.96

Business Acquisition, Fair Value of Loans Acquired [Table Text Block]

	Acquired		Acquired		Acquired
	Impaired		Non-Impaired		Total Loans
Commercial Loans:
Commercial & industrial	$	878,000	$	163,316,000	$	164,194,000
Commercial real estate		12,973,000		378,016,000		390,989,000
Construction & development		1,289,000		33,726,000		35,015,000
Total Commercial Loans	$	15,140,000	$	575,058,000	$	590,198,000
Consumer Loans:
Residential mortgages	$	9,694,000	$	216,653,000	$	226,347,000
Instalment		167,000		61,657,000		61,824,000
Home equity lines		288,000		52,054,000		52,342,000
Construction		76,000		12,875,000		12,951,000
Total Consumer Loans	$	10,225,000	$	343,239,000	$	353,464,000
Total Loans	$	25,365,000	$	918,297,000	$	943,662,000

Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Table Text Block]

	Acquired Impaired
Contractually required payments	$	44,936,000
Nonaccretable difference		17,057,000
Expected cash flows		27,879,000
Accretable yield		2,514,000
Carrying balance	$	25,365,000