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Borrowings - Narrative (Details) - USD ($)
Jun. 30, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Percent of assets 25.00%  
Maximum borrowing capacity $ 3,850,000,000 $ 3,810,000,000
Percent outstanding advances 100.00%  
Unsecured credit facility, maximum borrowing capacity $ 95,000,000.0 91,000,000.0
Advances from Federal Home Loan Banks 695,000,000 $ 821,280,000
Unused funds 3,130,000,000  
Federal funds purchased $ 0  
Percent of qualifying assets 95.00%  
Securities pledged as collateral $ 0  
Weighted Average    
Debt Instrument [Line Items]    
FHLB advances, interest rate 1.98% 1.78%
Minimum    
Debt Instrument [Line Items]    
FHLB advances, interest rate 1.35%  
Maximum    
Debt Instrument [Line Items]    
FHLB advances, interest rate 2.51%  
Mortgage Loans on Real Estate    
Debt Instrument [Line Items]    
Pledged as collateral $ 6,940,000,000 $ 6,010,000,000.00
Qualifying Loans    
Debt Instrument [Line Items]    
Asset balance used to determine maximum borrowing capacity from federal reserve bank 917,100,000  
Securities Investment    
Debt Instrument [Line Items]    
Total available borrowing capacity 736,300,000 786,600,000
Amount outstanding $ 0 0
Other assets    
Debt Instrument [Line Items]    
Investment in common trust securities   $ 3,900,000