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Borrowings - Narrative (Details) - USD ($)
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Percent of assets 25.00%  
Maximum amount available $ 3,810,000,000 $ 3,540,000,000
Percent outstanding advances 100.00%  
Unsecured credit facility, maximum borrowing capacity $ 91,000,000  
Federal Home Loan Bank Advances 821,300,000 1,160,000,000
Unused funds 2,970,000,000  
Federal funds purchased 0 69,900,000
Advances from Federal Home Loan Banks $ 821,280,000  
Percent of qualifying assets 95.00%  
Carrying values of securities pledged as collateral $ 354,600,000 359,200,000
Mortgage loans on real estate    
Debt Instrument [Line Items]    
Collateral pledged 6,010,000,000 4,910,000,000
Securities investment    
Debt Instrument [Line Items]    
Carrying values of securities pledged as collateral 0  
Total available borrowing capacity 786,600,000 732,000,000
Amount outstanding 0 $ 0
Qualifying loans    
Debt Instrument [Line Items]    
Asset balance used to determine maximum borrowing capacity from federal reserve bank $ 1,000,000,000  
Weighted Average [Member]    
Debt Instrument [Line Items]    
FHLB advances, interest rate at period end 1.78% 1.63%
Minimum    
Debt Instrument [Line Items]    
FHLB advances, interest rate at period end 1.06%  
Maximum    
Debt Instrument [Line Items]    
FHLB advances, interest rate at period end 2.47%