Basis of presentation (Policies)	3 Months Ended
Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	The accompanying unaudited condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP) for interim financial information, the instructions to SEC Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In preparing the unaudited condensed consolidated financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the balance sheet and the reported amounts of revenues and expenses for the period. Actual results could differ significantly from those estimates.
Recent accounting pronouncements	Recent accounting pronouncements. Segment Reporting. In November 2023, the FASB issued ASU No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures to improve reportable segment disclosure requirements, primarily through enhanced disclosure requirements of significant segment expenses. These amendments are effective for annual periods beginning after December 15, 2023 and interim periods within fiscal years beginning after December 15, 2024. Early adoption is permitted. These amendments apply on a retrospective basis. The Company is currently evaluating the impact of this amendment on the Company’s consolidated financial statements. Income Taxes. In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvement to Income Tax Disclosures to enhance the transparency and decision usefulness of income tax disclosures. The amendments are effective for annual periods beginning after December 15, 2024. These amendments apply on a prospective basis with a retrospective option. Early adoption is permitted. The Company is currently evaluating the impact of this amendment on the Company’s consolidated financial statements. Climate-related disclosures. In March 2024, the Securities and Exchange Commission (SEC) issued final climate-related disclosure rules under SEC Release No. 33-11275, The Enhancement and Standardization of Climate-Related Disclosures for Investors (climate disclosure rules). The rules will require annual disclosure of material greenhouse gas emissions as well as disclosure of governance, risk management and strategy related to material climate-related risks. In addition, the rules require (i) financial statement impacts of severe weather events and other natural conditions; (ii) a roll forward of carbon offset and renewable energy credit balances if material to the Company’s plan to achieve climate-related targets or goals; and (iii) material impacts on estimates and assumptions in the financial statements. The disclosure requirements will begin phasing in for annual periods beginning with calendar year 2025. The Company is currently evaluating the final rule to determine its impact on the Company’s consolidated financial statements. In April 2024, the SEC voluntarily stayed implementation of its climate disclosure rules pending completion of judicial review by the Court of Appeals for the Eighth Circuit.
Fair value measurements	The following are descriptions of the valuation methodologies used for assets and liabilities recorded at fair value and for estimating fair value for financial instruments not carried at fair value: Investment securities. The fair value of ASB’s investment securities is determined quarterly through pricing obtained from independent third-party pricing services or from brokers not affiliated with the trade. Non-binding broker quotes are infrequent and generally occur for new securities that are settled close to the month-end pricing date. The third-party pricing vendors ASB uses for pricing its securities are reputable firms that provide pricing services on a global basis and have processes in place to ensure quality and control. The third-party pricing services use a variety of methods to determine the fair value of securities that fall under Level 2 of ASB’s fair value measurement hierarchy. Among the considerations are quoted prices for similar securities in an active market, yield spreads for similar trades, adjustments for liquidity, size, collateral characteristics, historic and generic prepayment speeds, and other observable market factors. To enhance the robustness of the pricing process, ASB will on a quarterly basis compare its standard third-party vendor’s price with that of another third-party vendor. If the prices are within an acceptable tolerance range, the price of the standard vendor will be accepted. If the variance is beyond the tolerance range, an evaluation will be conducted by ASB and a challenge to the price may be made. Fair value in such cases will be based on the value that best reflects the data and observable characteristics of the security. In all cases, the fair value used will have been independently determined by a third-party pricing vendor or non-affiliated broker. The fair value of the mortgage revenue bonds is estimated using a discounted cash flow model to calculate the present value of future principal and interest payments and, therefore is classified within Level 3 of the valuation hierarchy. Loans held for sale. Residential and commercial loans are carried at the lower of cost or market and are valued using market observable pricing inputs, which are derived from third party loan sales and, therefore, are classified within Level 2 of the valuation hierarchy. Loans held for investment. Fair value of loans held for investment is derived using a discounted cash flow approach which includes an evaluation of the underlying loan characteristics. The valuation model uses loan characteristics which includes product type, maturity dates and the underlying interest rate of the portfolio. This information is input into the valuation models along with various forecast valuation assumptions including prepayment forecasts, to determine the discount rate. These assumptions are derived from internal and third party sources. Since the valuation is derived from model-based techniques, ASB includes loans held for investment within Level 3 of the valuation hierarchy. Collateral dependent loans. Collateral dependent loans have been adjusted to fair value. When a loan is identified as collateral dependent, the Company measures the impairment using the current fair value of the collateral, less selling costs. Depending on the characteristics of a loan, the fair value of collateral is generally estimated by obtaining external appraisals, but in some cases, the value of the collateral may be estimated as having little or no value. Non-real estate collateral may be valued using an appraisal, net book value per the borrower’s financial statements, or aging reports, adjusted or discounted based on management’s historical knowledge, changes in market conditions from the time of the valuation and management’s expertise and knowledge of the client and client’s business, resulting in a Level 3 fair value classification. If it is determined that the value of the collateral dependent loan is less than its recorded investment, the Company recognizes this impairment and adjusts the carrying value of the loan to fair value through the allowance for credit losses. Real estate acquired in settlement of loans. Foreclosed assets are initially measured at fair value (less estimated costs to sell) and subsequently measured at the lower of the carrying value or fair value less selling costs. Fair values are generally based upon appraisals or independent market prices that are periodically updated subsequent to classification as real estate owned. Such adjustments typically result in a Level 3 classification of the inputs for determining fair value. ASB estimates the fair value of collateral-dependent loans and real estate owned using the sales comparison approach. Mortgage servicing rights. MSRs are capitalized at fair value based on market data at the time of sale and accounted for in subsequent periods at the lower of amortized cost or fair value. MSRs are evaluated for impairment at each reporting date. ASB's MSRs are stratified based on predominant risk characteristics of the underlying loans including loan type and note rate. For each stratum, fair value is calculated by discounting expected net income streams using discount rates that reflect industry pricing for similar assets. Expected net income streams are estimated based on industry assumptions regarding prepayment expectations and income and expenses associated with servicing residential mortgage loans for others. Impairment is recognized through a valuation allowance for each stratum when the carrying amount exceeds fair value, with any associated provision recorded as a component of loan servicing fees included in “Revenues - bank” in the consolidated statements of income. A direct write-down is recorded when the recoverability of the valuation allowance is deemed to be unrecoverable. ASB compares the fair value of MSRs to an estimated value calculated by an independent third-party. The third-party relies on both published and unpublished sources of market related assumptions and its own experience and expertise to arrive at a value. ASB uses the third-party value only to assess the reasonableness of its own estimate. ASB includes MSRs within Level 3 of the valuation hierarchy. Time certificates. The fair value of fixed-maturity certificates of deposit was estimated by discounting the future cash flows using the rates currently offered for FHLB advances of similar remaining maturities. Deposit liabilities are classified in Level 2 of the valuation hierarchy. Other borrowings. For advances, repurchase agreements and other bank borrowings, fair value is estimated using quantitative discounted cash flow models that require the use of interest rate inputs that are currently offered for advances, repurchase agreements and other bank borrowings of similar remaining maturities. The majority of market inputs are actively quoted and can be validated through external sources, including broker market transactions and third party pricing services. Long-term debt—other than bank. Fair value of fixed-rate long-term debt—other than bank was obtained from third-party financial services providers based on the current rates offered for debt of the same or similar remaining maturities and from discounting the future cash flows using the current rates offered for debt of the same or similar risks, terms, and remaining maturities. The carrying amount of floating rate long-term debt—other than bank approximated fair value because of the short-term interest reset periods. Long-term debt—other than bank is classified in Level 2 of the valuation hierarchy. Interest rate lock commitments (IRLCs). The estimated fair value of commitments to originate residential mortgage loans for sale is based on quoted prices for similar loans in active markets. IRLCs are classified as Level 2 measurements. Forward sales commitments. To be announced (TBA) mortgage-backed securities forward commitments are classified as Level 1, and consist of publicly-traded debt securities for which identical fair values can be obtained through quoted market prices in active exchange markets. The fair values of ASB’s best efforts and mandatory delivery loan sale commitments are determined using quoted prices in the marketplace that are observable and are classified as Level 2 measurements. Interest rate swaps. The Company measures its interest rate swaps at fair value. The fair values of the Company's interest rate swaps are based on the estimated amounts that the Company would receive or pay to terminate the contracts at the reporting date and are determined using interest rate pricing models and interest rate related observable inputs. The fair values of the Company's interest rate swaps are classified as a Level 2 measurements.

Basis of presentation (Policies)

Recent accounting pronouncements.

Segment Reporting. In November 2023, the FASB issued ASU No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures to improve reportable segment disclosure requirements, primarily through enhanced disclosure requirements of significant segment expenses. These amendments are effective for annual periods beginning after December 15, 2023 and interim periods within fiscal years beginning after December 15, 2024. Early adoption is permitted. These amendments apply on a retrospective basis. The Company is currently evaluating the impact of this amendment on the Company’s consolidated financial statements.

Income Taxes. In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvement to Income Tax Disclosures to enhance the transparency and decision usefulness of income tax disclosures. The amendments are effective for annual periods beginning after December 15, 2024. These amendments apply on a prospective basis with a retrospective option. Early adoption is permitted. The Company is currently evaluating the impact of this amendment on the Company’s consolidated financial statements.

Climate-related disclosures. In March 2024, the Securities and Exchange Commission (SEC) issued final climate-related disclosure rules under SEC Release No. 33-11275, The Enhancement and Standardization of Climate-Related Disclosures for Investors (climate disclosure rules). The rules will require annual disclosure of material greenhouse gas emissions as well as disclosure of governance, risk management and strategy related to material climate-related risks. In addition, the rules require (i) financial statement impacts of severe weather events and other natural conditions; (ii) a roll forward of carbon offset and renewable energy credit balances if material to the Company’s plan to achieve climate-related targets or goals; and (iii) material impacts on estimates and assumptions in the financial statements. The disclosure requirements will begin phasing in for annual periods beginning with calendar year 2025. The Company is currently evaluating the final rule to determine its impact on the Company’s consolidated financial statements. In April 2024, the SEC voluntarily stayed implementation of its climate disclosure rules pending completion of judicial review by the Court of Appeals for the Eighth Circuit.

Investment securities. The fair value of ASB’s investment securities is determined quarterly through pricing obtained from independent third-party pricing services or from brokers not affiliated with the trade. Non-binding broker quotes are infrequent and generally occur for new securities that are settled close to the month-end pricing date. The third-party pricing vendors ASB uses for pricing its securities are reputable firms that provide pricing services on a global basis and have processes in place to ensure quality and control. The third-party pricing services use a variety of methods to determine the fair value of securities that fall under Level 2 of ASB’s fair value measurement hierarchy. Among the considerations are quoted prices for similar securities in an active market, yield spreads for similar trades, adjustments for liquidity, size, collateral characteristics, historic and generic prepayment speeds, and other observable market factors.

To enhance the robustness of the pricing process, ASB will on a quarterly basis compare its standard third-party vendor’s price with that of another third-party vendor. If the prices are within an acceptable tolerance range, the price of the standard vendor will be accepted. If the variance is beyond the tolerance range, an evaluation will be conducted by ASB and a challenge to the price may be made. Fair value in such cases will be based on the value that best reflects the data and observable characteristics of the security. In all cases, the fair value used will have been independently determined by a third-party pricing vendor or non-affiliated broker.

The fair value of the mortgage revenue bonds is estimated using a discounted cash flow model to calculate the present value of future principal and interest payments and, therefore is classified within Level 3 of the valuation hierarchy.

Loans held for sale. Residential and commercial loans are carried at the lower of cost or market and are valued using market observable pricing inputs, which are derived from third party loan sales and, therefore, are classified within Level 2 of the valuation hierarchy.

Loans held for investment. Fair value of loans held for investment is derived using a discounted cash flow approach which includes an evaluation of the underlying loan characteristics. The valuation model uses loan characteristics which includes product type, maturity dates and the underlying interest rate of the portfolio. This information is input into the valuation models along with various forecast valuation assumptions including prepayment forecasts, to determine the discount rate. These assumptions are derived from internal and third party sources. Since the valuation is derived from model-based techniques, ASB includes loans held for investment within Level 3 of the valuation hierarchy.

Collateral dependent loans. Collateral dependent loans have been adjusted to fair value. When a loan is identified as collateral dependent, the Company measures the impairment using the current fair value of the collateral, less selling costs. Depending on the characteristics of a loan, the fair value of collateral is generally estimated by obtaining external appraisals, but in some cases, the value of the collateral may be estimated as having little or no value. Non-real estate collateral may be valued using an appraisal, net book value per the borrower’s financial statements, or aging reports, adjusted or discounted based on management’s historical knowledge, changes in market conditions from the time of the valuation and management’s expertise and knowledge of the client and client’s business, resulting in a Level 3 fair value classification. If it is determined that the value of the collateral dependent loan is less than its recorded investment, the Company recognizes this impairment and adjusts the carrying value of the loan to fair value through the allowance for credit losses.

Real estate acquired in settlement of loans. Foreclosed assets are initially measured at fair value (less estimated costs to sell) and subsequently measured at the lower of the carrying value or fair value less selling costs. Fair values are generally based upon appraisals or independent market prices that are periodically updated subsequent to classification as real estate owned. Such adjustments typically result in a Level 3 classification of the inputs for determining fair value. ASB estimates the fair value of collateral-dependent loans and real estate owned using the sales comparison approach.

Mortgage servicing rights. MSRs are capitalized at fair value based on market data at the time of sale and accounted for in subsequent periods at the lower of amortized cost or fair value. MSRs are evaluated for impairment at each reporting date. ASB's MSRs are stratified based on predominant risk characteristics of the underlying loans including loan type and note rate. For each stratum, fair value is calculated by discounting expected net income streams using discount rates that reflect industry pricing for similar assets. Expected net income streams are estimated based on industry assumptions regarding prepayment expectations and income and expenses associated with servicing residential mortgage loans for others. Impairment is recognized through a valuation allowance for each stratum when the carrying amount exceeds fair value, with any associated provision recorded as a component of loan servicing fees included in “Revenues - bank” in the consolidated statements of income. A direct write-down is recorded when the recoverability of the valuation allowance is deemed to be unrecoverable. ASB compares the fair value of MSRs to an estimated value calculated by an independent third-party. The third-party relies on

both published and unpublished sources of market related assumptions and its own experience and expertise to arrive at a value. ASB uses the third-party value only to assess the reasonableness of its own estimate. ASB includes MSRs within Level 3 of the valuation hierarchy.

Time certificates. The fair value of fixed-maturity certificates of deposit was estimated by discounting the future cash flows using the rates currently offered for FHLB advances of similar remaining maturities. Deposit liabilities are classified in Level 2 of the valuation hierarchy.

Other borrowings. For advances, repurchase agreements and other bank borrowings, fair value is estimated using quantitative discounted cash flow models that require the use of interest rate inputs that are currently offered for advances, repurchase agreements and other bank borrowings of similar remaining maturities. The majority of market inputs are actively quoted and can be validated through external sources, including broker market transactions and third party pricing services.

Long-term debt—other than bank. Fair value of fixed-rate long-term debt—other than bank was obtained from third-party financial services providers based on the current rates offered for debt of the same or similar remaining maturities and from discounting the future cash flows using the current rates offered for debt of the same or similar risks, terms, and remaining maturities. The carrying amount of floating rate long-term debt—other than bank approximated fair value because of the short-term interest reset periods. Long-term debt—other than bank is classified in Level 2 of the valuation hierarchy.

Interest rate lock commitments (IRLCs). The estimated fair value of commitments to originate residential mortgage loans for sale is based on quoted prices for similar loans in active markets. IRLCs are classified as Level 2 measurements.

Forward sales commitments. To be announced (TBA) mortgage-backed securities forward commitments are classified as Level 1, and consist of publicly-traded debt securities for which identical fair values can be obtained through quoted market prices in active exchange markets. The fair values of ASB’s best efforts and mandatory delivery loan sale commitments are determined using quoted prices in the marketplace that are observable and are classified as Level 2 measurements.

Interest rate swaps. The Company measures its interest rate swaps at fair value. The fair values of the Company's interest rate swaps are based on the estimated amounts that the Company would receive or pay to terminate the contracts at the reporting date and are determined using interest rate pricing models and interest rate related observable inputs. The fair values of the Company's interest rate swaps are classified as a Level 2 measurements.