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Borrowed Funds - Additional Information (Detail)
1 Months Ended 12 Months Ended
May 31, 2015
USD ($)
Dec. 31, 2017
USD ($)
Instrument
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Debt Instrument [Line Items]        
Payments to acquire federal home loan bank stock     $ 10,000,000  
Federal home loan bank maturity period     30 days  
Securities sold under agreements to repurchase   $ 1,249,370,000 $ 1,249,370,000 $ 1,400,000,000
Accrued interest payable   197,000 197,000 $ 80,000
FHLB [Member]        
Debt Instrument [Line Items]        
Issuance of letter of credit   $ 300,000,000 $ 300,000,000  
Letter of credit expiration date     Jan. 31, 2018  
Number of letter of credit | Instrument   3    
Federal home loan bank amount outstanding   $ 0 $ 0  
Letter of Credit [Member] | FHLB [Member]        
Debt Instrument [Line Items]        
Borrowing capacity   2,000,000,000 $ 2,000,000,000  
Wells Fargo Bank 1.25% due 2012 [Member]        
Debt Instrument [Line Items]        
Interest rate based on LIBOR     1.00 percent above LIBOR  
Borrowing capacity   50,000,000 $ 50,000,000  
Debt instrument, interest rate based on prime lending rate     1.75 percent below the prime  
Revolving line of credit, commitment fee for unused portion     0.30%  
Revolving line of credit outstanding amount   0 $ 0  
Reduced Borrowing Limit [Member] | Letter of Credit [Member] | FHLB [Member]        
Debt Instrument [Line Items]        
Borrowing capacity   $ 1,700,000,000 $ 1,700,000,000  
Marquette [Member]        
Debt Instrument [Line Items]        
Long-term debt $ 89,971,000      
Acquisition date May 31, 2015      
Marquette Capital Trust I, Marquette Capital Trust II, Marquette Capital Trust III And Marquette Capital Trust IV [Member] | Marquette [Member]        
Debt Instrument [Line Items]        
Long-term debt $ 103,100,000      
Long-term debt acquired at fair value $ 65,500,000      
Trust Preferred Securities [Member] | Marquette Capital Trust I, Marquette Capital Trust III And Marquette Capital Trust IV [Member] | Marquette [Member]        
Debt Instrument [Line Items]        
Interest rate based on LIBOR     LIBOR rate with spreads ranging from 133 basis points to 160 basis points  
Preferred securities maturity date range, start     Jan. 31, 2036  
Preferred securities maturity date range, end     Sep. 30, 2036  
Minimum [Member] | Trust Preferred Securities [Member] | Marquette Capital Trust I, Marquette Capital Trust III And Marquette Capital Trust IV [Member] | Marquette [Member]        
Debt Instrument [Line Items]        
Interest rate of trust preferred securities 1.33%      
Maximum [Member] | Trust Preferred Securities [Member] | Marquette Capital Trust I, Marquette Capital Trust III And Marquette Capital Trust IV [Member] | Marquette [Member]        
Debt Instrument [Line Items]        
Interest rate of trust preferred securities 1.60%