Commitments, Off-Balance Sheet Arrangements, Legal Proceedings and Regulatory Matters (Details Textuals)	Jan. 11, 2016 USD ($) complaint	May 22, 2015 USD ($) branch loan_production_office	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 06, 2017 complaint	Aug. 30, 2017 complaint	Jul. 28, 2017 complaint
Commitments, Off-Balance Sheet Arrangements, and Contingent Liabilities (Textuals) [Line Items]
Valuation Allowances and Reserves, Balance			$ 26,000,000	$ 24,000,000
Number of complaints filed under legal proceedings | complaint	2
Conciliation Agreement, Commitment Term		3 years
Conciliation Agreement, Mortgage Loan Commitment		$ 196,000,000
Conciliation Agreement, Number of Branch Opening Commitments | branch		1
Conciliation Agreement, Number of Loan Production Offices, Commitment | loan_production_office		4
Loans Repurchased Under Make Whole Requests			0	1,000,000
Loans Sold To Outside Investors Original Amount			10,000,000,000
Loans Sold To Outside Investors Remaining Outstanding Amount			6,000,000,000
Residential Mortgage Loans Sold With Recourse Risk			36,000,000	44,000,000
Residential Mortgage Loans Sold With Credit Recourse Risk			69,000,000	73,000,000
The Official Committee of Unsecured Creditors of World Marketing Chicago LLC v Associated Bank [Member]
Commitments, Off-Balance Sheet Arrangements, and Contingent Liabilities (Textuals) [Line Items]
Loss Contingency, Damages Sought, Value	$ 6,000,000
Loss Contingency, Damages Sought, Value, Cash	4,000,000
American Funds Service Company v Associated Bank [Member]
Commitments, Off-Balance Sheet Arrangements, and Contingent Liabilities (Textuals) [Line Items]
Loss Contingency, Damages Sought, Value	600,000
Loss Contingency, Damages Sought, Value, Cash	$ 540,000
Schumel et al v. Bank Mutual Corporation et al, and Paquin et al v. Bank Mutual Corporation et al [Domain]
Commitments, Off-Balance Sheet Arrangements, and Contingent Liabilities (Textuals) [Line Items]
Number of complaints filed under legal proceedings | complaint							2
Wollenburg et al. v. Bank Mutual Corporation et al. [Member]
Commitments, Off-Balance Sheet Arrangements, and Contingent Liabilities (Textuals) [Line Items]
Number of complaints filed under legal proceedings | complaint						1
Parshall et al. v. Bank Mutual Corporation et al. [Member]
Commitments, Off-Balance Sheet Arrangements, and Contingent Liabilities (Textuals) [Line Items]
Number of complaints filed under legal proceedings | complaint					1
UnconsolidatedProjectsLowIncomeHousing [Member]
Commitments, Off-Balance Sheet Arrangements, and Contingent Liabilities (Textuals) [Line Items]
Other Investments			142,000,000	147,000,000
Commitments Under Unconsolidated Projects			111,000,000	119,000,000
PrincipalInvestmentCommitment [Member]
Commitments, Off-Balance Sheet Arrangements, and Contingent Liabilities (Textuals) [Line Items]
Other Investments			$ 24,000,000	$ 23,000,000