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Borrowings - Narrative (Details) - USD ($)
Mar. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Percent of assets 25.00%  
Maximum borrowing capacity $ 3,360,000,000 $ 3,380,000,000
Percent outstanding advances 100.00%  
Advances from FHLB $ 703,850,000 754,290,000
Unused funds $ 2,640,000,000  
Percent of qualifying assets 95.00%  
Putable    
Debt Instrument [Line Items]    
Advances from FHLB $ 20,100,000 $ 20,200,000
Weighted Average    
Debt Instrument [Line Items]    
FHLB advances, interest rate 1.29% 1.22%
Minimum    
Debt Instrument [Line Items]    
FHLB advances, interest rate 0.84%  
Maximum    
Debt Instrument [Line Items]    
FHLB advances, interest rate 2.02%  
Mortgage Loans on Real Estate    
Debt Instrument [Line Items]    
Pledged as collateral $ 5,500,000,000 $ 5,530,000,000
Qualifying Loans    
Debt Instrument [Line Items]    
Asset balance used to determine maximum borrowing capacity from federal reserve bank 651,700,000  
Securities Investment    
Debt Instrument [Line Items]    
Asset balance used to determine maximum borrowing capacity from federal reserve bank 5,700,000  
Amount outstanding $ 0 $ 0