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Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value $ 0  
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0  
Held to Maturity, More than 12 Months, Estimated Fair Value 900  
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 4  
Held to Maturity, Total Estimated Fair Value 900  
Held-to-maturity Securities, Gross Unrealized Losses 4 $ 637
Available for Sale, Less than 12 Months, Estimated Fair Value 61,278  
Available for Sale, Less than 12 Months, Unrealized Losses 1,317  
Available for Sale, More than 12 Months, Estimated Fair Value 2,123,415  
Available for Sale, More than 12 Months, Unrealized Losses 7,901  
Available for Sale, Total Estimated Fair Value 2,184,693  
Available-for-sale Securities, Gross Unrealized Loss, before Tax 9,218 108,737
Residential mortgage-backed securities [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value 0  
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0  
Held to Maturity, More than 12 Months, Estimated Fair Value 900  
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 4  
Held to Maturity, Total Estimated Fair Value 900  
Held-to-maturity Securities, Gross Unrealized Losses 4 85
Available for Sale, Less than 12 Months, Estimated Fair Value 12,258  
Available for Sale, Less than 12 Months, Unrealized Losses 38  
Available for Sale, More than 12 Months, Estimated Fair Value 116,919  
Available for Sale, More than 12 Months, Unrealized Losses 1,365  
Available for Sale, Total Estimated Fair Value 129,177  
Available-for-sale Securities, Gross Unrealized Loss, before Tax 1,403 6,547
States and political subdivisions [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held-to-maturity Securities, Gross Unrealized Losses 0 552
Available for Sale, Less than 12 Months, Estimated Fair Value 49,020  
Available for Sale, Less than 12 Months, Unrealized Losses 1,279  
Available for Sale, More than 12 Months, Estimated Fair Value 186,554  
Available for Sale, More than 12 Months, Unrealized Losses 1,593  
Available for Sale, Total Estimated Fair Value 235,574  
Available-for-sale Securities, Gross Unrealized Loss, before Tax 2,872 72,690
Other Investments [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held-to-maturity Securities, Gross Unrealized Losses 0 0
Available-for-sale Securities, Gross Unrealized Loss, before Tax 0 0
US Treasury Securities [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Available for Sale, Less than 12 Months, Estimated Fair Value 0  
Available for Sale, Less than 12 Months, Unrealized Losses 0  
Available for Sale, More than 12 Months, Estimated Fair Value 1,819,942  
Available for Sale, More than 12 Months, Unrealized Losses 4,943  
Available for Sale, Total Estimated Fair Value 1,819,942  
Available-for-sale Securities, Gross Unrealized Loss, before Tax $ 4,943 $ 29,500