Commitments, Off-Balance Sheet Arrangements, Legal Proceedings and Regulatory Matters (Details Textuals) $ in Millions		6 Months Ended	12 Months Ended
	May 22, 2015 USD ($) branch loan_production_office	Jun. 30, 2018 USD ($)	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]
SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount		$ 26	$ 24
Conciliation Agreement, Commitment Term	3 years
Conciliation Agreement, Mortgage Loan Commitment	$ 196
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		1	$ 1
Loans Sold To Outside Investors Original Amount		$ 10,500
Loans Sold To Outside Investors Loss ReimbursementSettlement Paid		zero	negligible
Loans Sold To Outside Investors Remaining Outstanding Amount		$ 6,200
Residential Mortgage Loans Sold With Recourse Risk		39	$ 44
Residential Mortgage Loans Sold With Credit Recourse Risk		64	73
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		137	147
Commitments Under Unconsolidated Projects		99	119
PrincipalInvestmentCommitment [Member]
Commitments, Off-Balance Sheet Arrangements, and Contingent Liabilities (Textuals) [Line Items]
Other Investments		$ 25	$ 23