Business Combinations - Consideration and Narrative (Details) - American National [Member] $ / shares in Units, $ in Thousands	Apr. 01, 2026 USD ($) $ / shares shares
Business Combination, Consideration Transferred, Equity Interest [Line Items]
Effective Date of Acquisition	Apr. 01, 2026
Business Combination, Name of Acquiree	American National
Business Combination, Control Obtained, Description	Pursuant to the Merger Agreement, (i) American National merged with and into Associated Banc-Corp, with Associated Banc-Corp continuing as the surviving corporation, and (ii) immediately following such merger, American National Bank, a national banking association and wholly owned subsidiary of American National, merged with and into the Bank, with the Bank continuing as the surviving bank.
Business Combination, Voting Equity Interest Acquired, Percentage	100.00%
Business Combination, Description of Acquiree	American National operated 33 branches across Nebraska, Minnesota and Iowa, with a concentration in the Greater Omaha and Minneapolis / St. Paul metro markets.
Business Combination, Reason for Business Combination	As a result of the acquisition, the Corporation increased its deposit market share and will deliver its products and services to an expanded client base across attractive Midwest markets.
Business Combination, Consideration Transferred, Equity Interest | $	$ 594,100
Business Combination, Price Per Share | $ / shares	$ 25.86
Business Acquisition Equity Consideration Per Share Conversion Ratio	36.250
Stock Issued During Period, Shares, Acquisitions	22,975,382