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Fair Value Measurements (Narrative) (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Mortgage Loans in Process of Foreclosure, Amount $ 12,500  
Unused Commitments to Extend Credit 6,519,034 $ 6,379,941
Valuation Allowance for Impairment of Recognized Servicing Assets, Sales and Disposals 1,400  
Fair Value, Nonrecurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other assets 4,700  
Rabbi Trust [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other assets 1,600  
Alternative investments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other assets 25,000  
Alternative investments [Member] | Fair Value, Nonrecurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Unused Commitments to Extend Credit $ 14,500