Note 1 - Summary of Significant Accounting Policies (Detail) (USD $) In Millions	6 Months Ended
	Jun. 30, 2011	Sep. 30, 2010
Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block]	Basis of Presentation - The accompanying unaudited consolidated interim financial statements of First Midwest Bancorp, Inc. (the "Company"), a Delaware corporation, have been prepared pursuant to the rules and regulations of the U.S. Securities and Exchange Commission for quarterly reports on Form 10-Q and do not include certain information and footnote disclosures required by U.S. generally accepted accounting principles ("GAAP") for complete annual financial statements. Accordingly, these financial statements should be read in conjunction with the Company's 2010 Annual Report on Form 10-K ("2010 10-K"). The accompanying unaudited consolidated interim financial statements have been prepared in accordance with GAAP and reflect all adjustments that are, in the opinion of management, necessary for the fair presentation of the financial position and results of operations for the periods presented. All such adjustments are of a normal recurring nature. The results of operations for the quarter and six-month periods ended June 30, 2011 are not necessarily indicative of the results that may be expected for the year ending December 31, 2011. Certain reclassifications have been made to prior periods to conform to the current period presentation. In third quarter 2010, the Company acquired approximately $297.0 in loans, $23.7 million in OREO, and $121.5 million in cash and securities and assumed $215.2 million in core deposits and $246.6 million in time deposits of a former bank in an FDIC-assisted transaction. The transaction resulted in the recognition of goodwill of $7.9 million. The fair values initially assigned to the assets acquired and liabilities assumed were preliminary and subject to refinement for up to one year after the closing date of the acquisition as new information relative to closing date fair values became available. Subsequent to the acquisition date, the Company obtained specific information (including the completion of appraisals or other valuations) relating to the acquisition-date value of certain assets and liabilities acquired, which required an adjustment to those assets and liabilities and to goodwill. In accordance with accounting guidance applicable to business combinations, such adjustments were recognized as if they had happened as of the acquisition date. After considering this additional information, the estimated fair value of covered loans decreased $2.9 million, covered OREO decreased $7.3 million, the FDIC indemnification asset increased $6.9 million; and accrued interest payable and other liabilities decreased $8.7 million from that originally reported in the quarter ended September 30, 2010. These revised estimates resulted in a $5.4 million decrease in goodwill and other intangible assets. As of June 30, 2011, the Company finalized its purchase price allocation. GAAP requires management to make certain estimates and assumptions. Although these and other estimates and assumptions are based on the best available information, actual results could be materially different from these estimates.
Loans Acquired In FDIC Assisted Transaction		$ 297.0
OREO Acquired In FDIC Assisted Transaction		23.7
CashAndSecuritiesAcquiredInFDICAssistedTransaction		121.5
Core Deposits Acquired In FDIC Assisted Transaction		215.2
Time Deposits Acquired In FDIC Assisted Transaction		246.6
Goodwill Acquired In FDIC Assisted Transaction		7.9
Covered Loan Adjustment		2.9
Covered Other Real Estate Owned Adjustment		7.3
FDIC Indemnification Asset Adjustment		6.9
Accrued Interest Payable And Other Liabilities Adjustment		8.7
Goodwill And Other Intangible Assets Adjustment		$ 5.4
Consolidation, Policy [Policy Text Block]	Principles of Consolidation - The consolidated financial statements include the accounts and results of operations of the Company and its subsidiaries after elimination of all significant intercompany accounts and transactions. The Company owns an interest in certain variable interest entities ("VIE"s) as described in Note 22, "Variable Interest Entities," contained in the Company's 2010 10-K. A VIE is a partnership, limited liability company, trust, or other legal entity that does not have sufficient equity to finance its activities without additional subordinated financial support from other parties, or whose investors lack certain characteristics associated with owning a controlling financial interest. The VIEs are not consolidated in the Company's financial statements since the Company is not the primary beneficiary of any of the VIEs. The accounting policies related to loans and the allowance for credit losses are presented below. For a summary of all other significant accounting policies, please refer to Note 1, "Summary of Significant Accounting Policies," contained in the Company's 2010 10-K
Loans and Leases Receivable, Valuation, Policy [Policy Text Block]	Loans - Loans are carried at the principal amount outstanding, including certain net deferred loan origination fees. Loans held-for-sale are carried at the lower of aggregate cost or fair value and included in other assets in the Consolidated Statements of Condition. Interest income on loans is accrued based on principal amounts outstanding. Loan and lease origination fees, fees for commitments that are expected to be exercised, and certain direct loan origination costs are deferred and the net amount amortized over the estimated life of the related loans or commitments as a yield adjustment. Fees related to standby letters of credit, whose ultimate exercise is remote, are amortized into fee income over the estimated life of the commitment. Other credit-related fees are recognized as fee income when earned
Certain Loans and Debt Securities Acquired in Transfer, Recognizing Interest Income on Impaired Loans, Policy [Policy Text Block]	Purchased Impaired Loans - Purchased impaired loans are recorded at their estimated fair values on the respective purchase dates and are accounted for prospectively based on expected cash flows in accordance with applicable authoritative accounting guidance. No allowance for credit losses is recorded on these loans at the acquisition date. In determining the acquisition date fair value of purchased impaired loans, and in subsequent accounting, the Company generally aggregates purchased consumer loans and certain smaller balance commercial loans into pools of loans with common risk characteristics such as delinquency status, credit score, and internal risk rating. Larger balance commercial loans are usually accounted for on an individual basis. Expected future cash flows in excess of the fair value of loans at the purchase date ("accretable yield") are recorded as interest income over the life of the loans if the timing and amount of the future cash flows can be reasonably estimated. The non-accretable yield represents estimated losses in the portfolio and is equal to the difference between contractually required payments and the cash flows expected to be collected at acquisition. Subsequent to the purchase date, increases in cash flows for purchased impaired loans over those expected at the purchase date are recognized as interest income prospectively. The present value of any decreases in expected cash flows after the purchase date is recognized by recording a charge-off through the allowance for loan losses
Loans and Leases Receivable, Nonaccrual Loan and Lease Status, Policy [Policy Text Block]	Non-accrual loans - Generally, commercial loans and loans secured by real estate are placed on non-accrual status: (a) when either principal or interest payments become 90 days or more past due based on contractual terms unless the loan is sufficiently collateralized such that full repayment of both principal and interest is expected and is in the process of collection within a reasonable period; or (b) when an individual analysis of a borrower's creditworthiness indicates a credit should be placed on non-accrual status whether or not the loan is 90 days or more past due. When a loan is placed on non-accrual status, unpaid interest credited to income in the current year is reversed, and unpaid interest accrued in prior years is charged against the allowance for loan losses. Both principal and interest payments are applied to the principal on the loan. Future interest income may only be recorded on a cash basis after recovery of principal is reasonably assured. Non-accrual loans are returned to accrual status when the financial position of the borrower and other relevant factors indicate there is no longer doubt that the Company will collect all principal and interest due. Commercial loans and loans secured by real estate are generally charged-off when deemed uncollectible. A loss is recorded at that time if the net realizable value can be quantified and it is less than the associated principal and interest outstanding. Consumer loans that are not secured by real estate are subject to mandatory charge-off at a specified delinquency date and are usually not classified as non-accrual prior to being charged-off. Closed-end consumer loans, which include installment, automobile, and single payment loans are generally charged-off in full no later than the end of the month in which the loan becomes 120 days past due. Generally, purchased impaired loans are considered accruing loans. However, the timing and amount of future cash flows for some loans may not be reasonably estimable. Those loans were classified as non-accrual loans as of June 30, 2011, and interest income will not be recognized until the timing and amount of the future cash flows can be reasonably estimated
Loans and Leases Receivable, Troubled Debt Restructuring Policy [Policy Text Block]	Restructured Loans - In cases where a borrower experiences financial difficulties and the Company makes certain concessions or modifications to contractual terms, the loan is classified as a restructured loan. Restructured loans are loans for which the original contractual terms have been modified, including forgiveness of principal or interest, due to deterioration in the borrower's financial condition. Loans granted concessions or modifications are classified as restructured loans unless the modification is short-term, or results in only an insignificant delay or shortfall in the payments to be received. The Company's restructured loans are determined on a case-by-case basis in connection with ongoing loan collection processes. The allowance for loan losses on restructured loans is determined by discounting the restructured cash flows at the original effective rate of the loan before modification or is based on the underlying collateral value. The Company does not accrue interest on any restructured loan unless and until it believes collection of all principal and interest under the modified terms is reasonably assured. Generally, six months of consecutive payment performance by the borrower under the restructured terms is required before a restructured loan is returned to accrual status assuming the loan is restructured at reasonable market terms (e.g., not at below market terms). However, the period could vary depending upon the individual facts and circumstances of the loan. For a restructured loan to begin accruing interest, the borrower must demonstrate both some level of performance and the capacity to perform under the modified terms. A history of timely payments and adherence to financial covenants generally serve as sufficient evidence of the borrower's performance. An evaluation of the borrower's current creditworthiness is used to assess whether the borrower has the capacity to repay the loan under the modified terms. This evaluation includes an estimate of expected cash flows, evidence of strong financial position, and estimates of the value of collateral, if applicable
Impaired Financing Receivable, Policy [Policy Text Block]	Impaired Loans - Impaired loans consist of corporate non-accrual loans and restructured loans in accordance with applicable authoritative accounting guidance. With the exception of loans that were restructured and still accruing interest, a loan is considered impaired when, based on current information and events, it is probable that the Company will be unable to collect all contractual principal and interest due according to the terms of the loan agreement. Loans deemed to be impaired are classified as non-accrual and are exclusive of smaller homogeneous loans, such as home equity, installment, and 1-4 family mortgages. When a loan is designated as impaired, any subsequent principal and interest payments received are applied to the principal on the loan. Future interest income may only be recorded on a cash basis after recovery of principal is reasonably assured. Certain impaired loans with balances under a specified threshold are not individually evaluated for impairment. For all other impaired loans, impairment is measured by estimating the value of the loan based on the present value of expected future cash flows discounted at the loan's initial effective interest rate or the fair value of the underlying collateral less costs to sell, if repayment of the loan is collateral-dependent. The Company evaluates the collectability of both principal and interest when assessing the need for loss accrual. All impaired loans are included in non-performing assets. Purchased credit impaired loans are not reported as impaired loans provided that they continue to perform in accordance with expected cash flows
Loans and Leases Receivable, Past Due Status, Policy [Policy Text Block]	90-Day Past Due Loans - 90 days or more past due loans are loans for which principal or interest payments become 90 days or more past due, but that still accrue interest. The Company continues to accrue interest if it determines these loans are well secured and in the process of collection.
Loans and Leases Receivable, Allowance for Loan Losses Policy [Policy Text Block]	Allowance for Credit Losses - The allowance for credit losses is comprised of the allowance for loan losses and the reserve for unfunded commitments and is maintained by management at a level believed adequate to absorb estimated losses inherent in the existing loan portfolio. Determination of the allowance for credit losses is inherently subjective, as it requires significant estimates, including the amounts and timing of expected future cash flows on impaired loans, estimated losses on pools of homogeneous loans based on historical loss experience, consideration of current economic trends, and other factors, all of which may be susceptible to significant change. The allowance for loan losses takes into consideration such internal and external qualitative factors as changes in the nature, volume, size, and current risk characteristics of the loan portfolio; an assessment of individual problem loans; actual and anticipated loss experience; current economic conditions that affect the borrower's ability to pay; and other pertinent factors. Credit exposures deemed to be uncollectible are charged-off against the allowance for loan losses, while recoveries of amounts previously charged-off are credited to the allowance for loan losses. Additions to the allowance for loan losses are established through the provision for loan losses charged to expense. The amount charged to operating expense in any given period is dependent upon a number of factors including historic loan growth, changes in the composition of the loan portfolio, net charge-off levels, and the Company's assessment of the allowance for loan losses based on the methodology discussed below. The allowance for loan losses consists of (i) specific reserves established for probable losses on individual loans for which the recorded investment in the loan exceeds the value of the loan, (ii) an allowance based on historical credit loss experience for each loan category, and (iii) the impact of other internal and external qualitative factors. The specific reserves component of the allowance for loan losses is based on a periodic analysis of impaired loans exceeding a fixed dollar amount where the internal credit rating is at or below a predetermined classification, as well as other loans regardless of internal credit rating that management believes are subject to a higher risk of loss. The value of the loan is measured based on the present value of expected future cash flows, discounted at the loan's initial effective interest rate, or the fair value of the underlying collateral less costs to sell, if repayment of the loan is collateral-dependent. If the resulting amount is less than the recorded book value, the Company either establishes a valuation allowance (i.e., a specific reserve) as a component of the allowance for loan losses or charges-off the impaired balance if it determines that such amount is a confirmed loss. The component of the allowance for loan losses based on historical credit loss experience is determined using a loss migration analysis that examines actual loss experience over a rolling 8-quarter period and, for corporate loans, the related internal rating of loans charged-off. The loss migration analysis is performed quarterly and loss factors are updated regularly based on actual experience. The loss component based upon historical loss experience is then adjusted for management's estimate of those losses inherent in the loan portfolio that have yet to be manifested in historical charge-off experience. Management takes into consideration many internal and external qualitative factors when estimating this adjustment, including: Changes in the composition of the loan portfolio and trends in volume and terms of loans, as well as trends in delinquent and non-accrual loans that could indicate historical averages do not reflect current conditions; Changes in credit policies and procedures, including underwriting standards and collection, charge-off, and recovery practices not considered elsewhere in estimating credit losses; Changes in the experience, ability, and depth of credit management and other relevant staff; Changes in the quality of the Company's loan review system and Board oversight; The existence and effect of any concentration of credit, and changes in the level of concentrations, whether it is by market, loan type, or risk taking; Changes in the value of underlying collateral for collateral-dependent loans; Changes in the national and local economy that affect the collectability of the portfolio, including the condition of various market segments; and The effect of other external factors such as competition and legal and regulatory requirements on the level of estimated credit losses in the Company's existing portfolio. The Company also maintains a reserve for unfunded credit commitments, including letters of credit, to provide for the risk of loss inherent in these arrangements. The reserve for unfunded credit commitments is computed based on a loss migration analysis similar to that used to determine the allowance for loan losses, taking into consideration probabilities of future funding requirements. This reserve for unfunded commitments is included in other liabilities in the Consolidated Statements of Financial Condition. The establishment of the allowance for credit losses involves a high degree of judgment and includes a level of imprecision given the difficulty of identifying all of the factors impacting loan repayment and the timing of when losses actually occur. While management utilizes its best judgment and information available, the ultimate adequacy of the allowance for credit losses is dependent upon a variety of factors beyond the Company's control, including the performance of its loan portfolio, the economy, changes in interest rates and property values, and the interpretation by regulatory authorities of loan risk classifications. While each component of the allowance for credit losses is determined separately, the entire balance is available for the entire loan portfolio