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Borrowings - Narrative (Details) - USD ($)
Dec. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Percent of assets 25.00%  
Maximum amount available $ 3,850,000,000 $ 3,810,000,000
Percent outstanding advances 100.00%  
Unsecured credit facility, maximum borrowing capacity $ 95,000,000.0  
Federal Home Loan Bank Advances 625,000,000.0 821,300,000
Advances with fixed interest rates until maturity 475,000,000.0  
Advances with variable interest rates 150,000,000.0  
Unused funds 3,190,000,000  
Advances from Federal Home Loan Banks $ 625,000,000  
Percent of qualifying assets 95.00%  
Carrying values of securities pledged as collateral $ 340,900,000 354,600,000
Mortgage loans on real estate    
Debt Instrument [Line Items]    
Collateral pledged 6,760,000,000 6,010,000,000.00
Securities investment    
Debt Instrument [Line Items]    
Carrying values of securities pledged as collateral 0  
Total available borrowing capacity 740,600,000 786,600,000
Amount outstanding 0 $ 0
Qualifying loans    
Debt Instrument [Line Items]    
Asset balance used to determine maximum borrowing capacity from federal reserve bank $ 947,200,000  
Weighted Average [Member]    
Debt Instrument [Line Items]    
FHLB advances, interest rate at period end 1.84% 1.78%
Minimum    
Debt Instrument [Line Items]    
FHLB advances, interest rate at period end 1.35%  
Maximum    
Debt Instrument [Line Items]    
FHLB advances, interest rate at period end 2.39%