Recent Accounting Pronouncements	3 Months Ended
Mar. 31, 2019
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Accounting Pronouncements	RECENT ACCOUNTING PRONOUNCEMENTS Adopted Accounting Pronouncements Leases: In February of 2016, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2016-02 to increase transparency and comparability across entities for leasing arrangements. This guidance requires lessees to recognize assets and liabilities for most leases. For lessors, this guidance modifies the lease classification criteria and the accounting for sales-type and direct financing leases. In addition, this guidance clarifies criteria for the determination of whether a contract is or contains a lease. This guidance is effective for annual and interim periods beginning after December 15, 2018. The Company adopted this guidance on January 1, 2019, which resulted in the recognition of $143.6 million of right-of-use assets and additional associated lease liabilities for its operating leases. The amount of right-of-use assets and associated lease liabilities recorded upon adoption was based on the present value of future minimum lease payments, the amount of which depended on the population of leases in effect at the date of adoption. This guidance also applies to the Company's net investment in direct financing leases, which is included in loans, but did not have a material impact. The Company has elected certain practical expedients contained in this guidance, which, among other provisions, allowed the Company to not reassess the historical lease classification, initial direct costs, or existing contracts for the inclusion of leases. The Company has also elected the practical expedients for the use of hindsight in determining the lease term and the right-of-use assets, as well as an election not to apply the recognition requirements of the guidance to leases with terms of 12 months or less. The application of hindsight practical expedient resulted in the determination that most renewal options would not be reasonably certain in determining the expected lease term. First Midwest Bank (the "Bank") entered into a sale-leaseback transaction in 2016 that resulted in a deferred gain. Upon adoption of this guidance, the remaining deferred gain of $47.3 million after tax was recognized immediately as a cumulative-effect adjustment to equity. For additional discussion of the sale-leaseback transaction, see Note 8 "Lease Obligations." The adoption of this guidance was applied retrospectively at the beginning of the period of adoption through a cumulative-effect adjustment and did not materially impact the Company's results of operations or liquidity, but did result in a material increase in assets, liabilities, and equity. Premium Amortization on Purchased Callable Debt Securities: In March of 2017, the FASB issued ASU 2017-08 that shortens the amortization period for the premium on certain purchased callable debt securities to the earliest call date. This guidance is effective for annual and interim periods beginning after December 15, 2018. The adoption of this guidance on January 1, 2019 did not materially impact the Company's financial condition, results of operations, or liquidity. Improvements to Nonemployee Share-based Payment Accounting: In June of 2018, the FASB issued ASU 2018-07 that aligns the measurement and classification guidance for share-based payments to nonemployees with the guidance for share-based payments to employees. This guidance is effective for annual and interim periods beginning after December 15, 2018. The adoption of this guidance on January 1, 2019 did not materially impact the Company's financial condition, results of operations, or liquidity. Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract: In August of 2018, the FASB issued ASU 2018-15 to reduce diversity in practice by clarifying when implementation costs are required to be capitalized in a cloud computing arrangement that is a service contract. This guidance is effective for annual and interim periods beginning after December 15, 2019. The early adoption of this guidance on January 1, 2019 did not materially impact the Company's financial condition, results of operations, or liquidity. Derivatives and Hedging, Inclusion of the Secured Overnight Financing Rate ("SOFR") Overnight Index Swap Rate as a Benchmark Interest Rate for Hedge Accounting Purposes: In October of 2018, the FASB issued ASU 2018-16 adding the overnight index swap rate based on the SOFR to the list of United States benchmark interest rates eligible for hedge accounting purposes. This guidance is effective for annual and interim periods beginning after December 15, 2018. The adoption of this guidance on January 1, 2019 did not materially impact the Company's financial condition, results of operations, or liquidity. Accounting Pronouncements Pending Adoption Measurement of Credit Losses on Financial Instruments: In June of 2016, the FASB issued ASU 2016-13 that will require entities to present financial assets measured at amortized cost at the net amount expected to be collected, considering an entity's current estimate of all expected credit losses. In addition, credit losses relating to available-for-sale debt securities will be required to be recorded through an allowance for credit losses, with changes in credit loss estimates recognized through current earnings. This guidance is effective for annual and interim periods beginning after December 15, 2019. Early adoption is permitted, but not for periods beginning before December 15, 2018. Management is evaluating the guidance and the impact to the Company's financial condition, results of operations, or liquidity. Accounting for Goodwill Impairment: In January of 2017, the FASB issued ASU 2017-04 that simplifies the accounting for goodwill impairment for all entities. The new guidance eliminates the requirement to calculate the implied fair value of goodwill using the second step of the quantitative two-step goodwill impairment model prescribed under current accounting guidance. Under the new guidance, if a reporting unit's carrying amount exceeds its fair value, an entity will record an impairment charge based on that difference. This guidance is effective for annual and interim goodwill impairment testing dates beginning after December 15, 2019. Early adoption is permitted for annual and interim goodwill impairment testing dates after January 1, 2017. Management does not expect the adoption of this guidance will materially impact the Company's financial condition, results of operations, or liquidity. Changes to the Disclosure Requirements for Fair Value Measurement: In August of 2018, the FASB issued ASU 2018-13 that eliminates, modifies, and adds to certain fair value measurement disclosure requirements associated with the three-tiered fair value hierarchy. This guidance is effective for annual and interim periods beginning after December 15, 2019. Early adoption is permitted. Management does not expect the adoption of this guidance will materially impact the Company's financial condition, results of operations, or liquidity. Changes to the Disclosure Requirements for Defined Benefit Plans: In August of 2018, the FASB issued ASU 2018-14 that makes minor changes and clarifications to the disclosure requirements for entities that sponsor defined benefit plans. This guidance is effective for annual and interim periods beginning after December 15, 2020. Early adoption is permitted. Management does not expect the adoption of this guidance will materially impact the Company's financial condition, results of operations, or liquidity.

RECENT ACCOUNTING PRONOUNCEMENTS

Adopted Accounting Pronouncements

Leases: In February of 2016, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2016-02 to increase transparency and comparability across entities for leasing arrangements. This guidance requires lessees to recognize assets and liabilities for most leases. For lessors, this guidance modifies the lease classification criteria and the accounting for sales-type and direct financing leases. In addition, this guidance clarifies criteria for the determination of whether a contract is or contains a lease. This guidance is effective for annual and interim periods beginning after December 15, 2018.

The Company adopted this guidance on January 1, 2019, which resulted in the recognition of $143.6 million of right-of-use assets and additional associated lease liabilities for its operating leases. The amount of right-of-use assets and associated lease liabilities recorded upon adoption was based on the present value of future minimum lease payments, the amount of which depended on the population of leases in effect at the date of adoption. This guidance also applies to the Company's net investment in direct financing leases, which is included in loans, but did not have a material impact.

The Company has elected certain practical expedients contained in this guidance, which, among other provisions, allowed the Company to not reassess the historical lease classification, initial direct costs, or existing contracts for the inclusion of leases. The Company has also elected the practical expedients for the use of hindsight in determining the lease term and the right-of-use assets, as well as an election not to apply the recognition requirements of the guidance to leases with terms of 12 months or less. The application of hindsight practical expedient resulted in the determination that most renewal options would not be reasonably certain in determining the expected lease term.

First Midwest Bank (the "Bank") entered into a sale-leaseback transaction in 2016 that resulted in a deferred gain. Upon adoption of this guidance, the remaining deferred gain of $47.3 million after tax was recognized immediately as a cumulative-effect adjustment to equity. For additional discussion of the sale-leaseback transaction, see Note 8 "Lease Obligations." The adoption of this guidance was applied retrospectively at the beginning of the period of adoption through a cumulative-effect adjustment and did not materially impact the Company's results of operations or liquidity, but did result in a material increase in assets, liabilities, and equity.

Premium Amortization on Purchased Callable Debt Securities: In March of 2017, the FASB issued ASU 2017-08 that shortens the amortization period for the premium on certain purchased callable debt securities to the earliest call date. This guidance is effective for annual and interim periods beginning after December 15, 2018. The adoption of this guidance on January 1, 2019 did not materially impact the Company's financial condition, results of operations, or liquidity.

Improvements to Nonemployee Share-based Payment Accounting: In June of 2018, the FASB issued ASU 2018-07 that aligns the measurement and classification guidance for share-based payments to nonemployees with the guidance for share-based payments to employees. This guidance is effective for annual and interim periods beginning after December 15, 2018. The adoption of this guidance on January 1, 2019 did not materially impact the Company's financial condition, results of operations, or liquidity.

Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract: In August of 2018, the FASB issued ASU 2018-15 to reduce diversity in practice by clarifying when implementation costs are required to be capitalized in a cloud computing arrangement that is a service contract. This guidance is effective for annual and interim periods beginning after December 15, 2019. The early adoption of this guidance on January 1, 2019 did not materially impact the Company's financial condition, results of operations, or liquidity.

Derivatives and Hedging, Inclusion of the Secured Overnight Financing Rate ("SOFR") Overnight Index Swap Rate as a Benchmark Interest Rate for Hedge Accounting Purposes: In October of 2018, the FASB issued ASU 2018-16 adding the overnight index swap rate based on the SOFR to the list of United States benchmark interest rates eligible for hedge accounting purposes. This guidance is effective for annual and interim periods beginning after December 15, 2018. The adoption of this guidance on January 1, 2019 did not materially impact the Company's financial condition, results of operations, or liquidity.

Accounting Pronouncements Pending Adoption

Measurement of Credit Losses on Financial Instruments: In June of 2016, the FASB issued ASU 2016-13 that will require entities to present financial assets measured at amortized cost at the net amount expected to be collected, considering an entity's current estimate of all expected credit losses. In addition, credit losses relating to available-for-sale debt securities will be required to be recorded through an allowance for credit losses, with changes in credit loss estimates recognized through current earnings. This guidance is effective for annual and interim periods beginning after December 15, 2019. Early adoption is permitted, but not for periods beginning before December 15, 2018. Management is evaluating the guidance and the impact to the Company's financial condition, results of operations, or liquidity.

Accounting for Goodwill Impairment: In January of 2017, the FASB issued ASU 2017-04 that simplifies the accounting for goodwill impairment for all entities. The new guidance eliminates the requirement to calculate the implied fair value of goodwill using the second step of the quantitative two-step goodwill impairment model prescribed under current accounting guidance. Under the new guidance, if a reporting unit's carrying amount exceeds its fair value, an entity will record an impairment charge based on that difference. This guidance is effective for annual and interim goodwill impairment testing dates beginning after December 15, 2019. Early adoption is permitted for annual and interim goodwill impairment testing dates after January 1, 2017. Management does not expect the adoption of this guidance will materially impact the Company's financial condition, results of operations, or liquidity.

Changes to the Disclosure Requirements for Fair Value Measurement: In August of 2018, the FASB issued ASU 2018-13 that eliminates, modifies, and adds to certain fair value measurement disclosure requirements associated with the three-tiered fair value hierarchy. This guidance is effective for annual and interim periods beginning after December 15, 2019. Early adoption is permitted. Management does not expect the adoption of this guidance will materially impact the Company's financial condition, results of operations, or liquidity.

Changes to the Disclosure Requirements for Defined Benefit Plans: In August of 2018, the FASB issued ASU 2018-14 that makes minor changes and clarifications to the disclosure requirements for entities that sponsor defined benefit plans. This guidance is effective for annual and interim periods beginning after December 15, 2020. Early adoption is permitted. Management does not expect the adoption of this guidance will materially impact the Company's financial condition, results of operations, or liquidity.