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Other Borrowings - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended
Nov. 01, 2017
Jan. 01, 2004
Sep. 30, 2018
Aug. 31, 2014
Debt Instrument [Line Items]        
Long-term capital lease obligation period, in years   25 years    
Long-term capital lease obligation renewal period, in years   10 years    
Anchor Bank (MN) [Member]        
Debt Instrument [Line Items]        
Remaining base term of lease 5 years      
Renewal option available for lease in years 10 years      
Capital lease obligation $ 1.5      
Senior Unsecured Notes [Member]        
Debt Instrument [Line Items]        
Senior unsecured notes       $ 175.0
Fixed rates       4.125%
Maturity Date     Aug. 15, 2024  
Subordinated Debentures [Member] | Anchor Bank (MN) [Member] | Subordinated Fixed-To-Floating Notes [Member]        
Debt Instrument [Line Items]        
Fixed rates 5.75%      
Maturity Date Oct. 30, 2025      
Value of subordinated fixed-to-floating notes assumed $ 12.0      
Subordinated debentures, maturity date Oct. 29, 2020      
Description of LIBOR Rate three-month LIBOR rate plus 4.356%      
LIBOR rate 4.356%      
Debt instrument start date Oct. 30, 2020