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DERIVATIVE FINANCIAL INSTRUMENTS AND HEDGING ACTIVITIES - Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Derivative [Line Items]    
Notional amount $ 3,810,420,000 $ 3,857,115,000
Cash collateral pledged 70,300,000  
Amortized cost of securities pledged as collateral to derivative counterparties 36,900,000  
Fair value of securities as pledged collateral to derivative counterparties 37,100,000  
Commercial counterparties    
Derivative [Line Items]    
Net asset position 1,300,000 1,000,000.0
Interest rate swaps    
Derivative [Line Items]    
Notional amount 8,500,000  
Cash collateral pledged 70,333,000 75,070,000
Interest rate swaps | Commercial counterparties    
Derivative [Line Items]    
Cash collateral pledged 0 0
Net asset position 103,800,000 159,000,000.0
Derivative liability, amount offset against collateral 2,600,000 0
Interest rate swaps | Institutional counterparties    
Derivative [Line Items]    
Cash collateral pledged 70,333,000 75,070,000
Derivative liability, amount offset against collateral 43,000,000.0 66,800,000
Not Designated as Hedging Instrument    
Derivative [Line Items]    
Notional amount 10,316,000 $ 40,099,000
Economic hedges    
Derivative [Line Items]    
Notional amount 3,800,000,000  
Credit valuation adjustments $ 2,500,000  
Economic hedges | Interest rate swap on tax advantaged economic development bond    
Derivative [Line Items]    
Fixed rate of interest 5.09%  
Maturity period 21 years  
Economic hedges | Interest rate swaps    
Derivative [Line Items]    
Notional amount $ 3,400,000,000  
Economic hedges | Risk participation agreements with dealer banks    
Derivative [Line Items]    
Notional amount 300,000,000  
Economic hedges | Forward sale commitments    
Derivative [Line Items]    
Notional amount $ 8,700,000