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Loans and Allowance for Loan Losses - Additional Information (Detail) - USD ($)
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Nov. 01, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]        
Commercial real estate $ 4,451,772,000 $ 3,259,998,000 $ 4,354,552,000  
Total loans 11,295,629,000 9,232,040,000 11,118,121,000  
Loan participations sold $ 412,800,000      
Loan placed on nonaccrual when past due, number of days 90 days      
Loan participations $ 820,200,000      
Loan participations retained $ 407,400,000      
Troubled debt restructuring term 6 months      
Minimum number of days for loan charge off to be recorded 120 days      
Maximum number of days for loan charge off to be recorded 180 days      
Nonaccrual period for loans 90 days      
Financing receivable TDR's included with non-accrual loans $ 34,000,000   34,000,000  
Financing receivable troubled debt restructurings specific reserves 4,800,000   5,700,000  
Unfunded commitments on TDRs 4,000,000      
Increase (decrease) in allowance for loan losses 0 3,400,000    
Allowance for loan losses charge-offs $ 0 0    
Number of days for a loan to be considered to be in payment default 90 days      
Outstanding loans including principal, interest, fees and penalties $ 240,900,000   235,900,000  
Accretion recorded as loan interest income 7,100,000 $ 7,300,000    
Financing receivable allowance for loan losses related to purchased loans 300,000   200,000  
Residential Real Estate [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Total loans 2,153,973,000   2,167,053,000  
Consumer [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Total loans 1,726,989,000   1,879,247,000  
Commercial Real Estate [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Total loans $ 4,451,772,000   4,354,552,000  
Credit Concentration Risk [Member] | Residential Real Estate [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Concentration Risk Percentage 10.00%      
Credit Concentration Risk [Member] | Non Residential Real Estate [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Concentration Risk Percentage 10.00%      
Maximum [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Value of small commercial loans on nonaccrual status or 90 days or more delinquent $ 250,000      
AnchorBank WI [Member] | Consumer [Member] | Student Loan [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Total loans     $ 68,200,000  
Loan participations sold 64,900,000      
AnchorBank WI [Member] | Consumer [Member] | Student Loan [Member] | Other Income [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Gain on sale of student loan $ 2,200,000      
AnchorBank WI [Member] | Maximum [Member] | Consumer [Member] | Student Loan [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Loan guarantee percentage 100.00%      
AnchorBank WI [Member] | Minimum [Member] | Consumer [Member] | Student Loan [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Loan guarantee percentage 97.00%      
Anchor Bank (MN) [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Commercial real estate       $ 864,400,000
Anchor Bank (MN) [Member] | Commercial Real Estate [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Percentage of risk-based capital 208.00%      
Regulatory guideline limit 300.00%