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Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities $ 1,114,930 $ 1,116,102
Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 1,022,419  
Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 92,511  
Credit Rating, AA [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 8,998  
Credit Rating, AA [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 8,998  
Credit Rating, A [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 420,166  
Credit Rating, A [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 362,821  
Credit Rating, A [Member] | Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 57,345  
Credit Rating, BBB [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 649,841  
Credit Rating, BBB [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 614,675  
Credit Rating, BBB [Member] | Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 35,166  
Credit Rating, BB [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 23,364  
Credit Rating, BB [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 23,364  
Credit Rating, CCC-C [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 12,561  
Credit Rating, CCC-C [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities $ 12,561