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Junior Subordinated Debentures (Narrative) (Detail)
9 Months Ended
Sep. 30, 2020
USD ($)
trust
Rate
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Subordinated Borrowing [Line Items]      
Percentage ownership interest in subsidiary trusts 100.00%    
Number of unconsolidated subsidiary trusts | trust 11    
Common securities, approximate percentage of junior subordinated debentures 3.00%    
Trust preferred securities, approximate percentage of junior subordinated debentures 97.00%    
Junior subordinated debentures $ 253,566,000 $ 253,566,000 $ 253,566,000
Period of deferred payment, not to exceed 60 months    
Junior Subordinated Debt      
Subordinated Borrowing [Line Items]      
Junior subordinated debentures $ 253,566,000    
Average contractual rate | Rate 2.44%    
Tier two risk based capital $ 245,500,000    
Interest rate swaps | Junior Subordinated Debt      
Subordinated Borrowing [Line Items]      
Derivative notional amount $ 210,000,000.0    
Hedge-adjusted contractual interest rate 3.99%