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Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value $ 1,499  
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1  
Held to Maturity, More than 12 Months, Estimated Fair Value 413  
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 2  
Held to Maturity, Total Estimated Fair Value 1,912  
Held-to-maturity Securities, Gross Unrealized Losses 3 $ 637
Available for Sale, Less than 12 Months, Estimated Fair Value 91,276  
Available for Sale, Less than 12 Months, Unrealized Losses 703  
Available for Sale, More than 12 Months, Estimated Fair Value 1,766,616  
Available for Sale, More than 12 Months, Unrealized Losses 2,221  
Available for Sale, Total Estimated Fair Value 1,857,892  
Available-for-sale Securities, Gross Unrealized Loss, before Tax 2,924 108,737
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 413  
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 2  
Held to Maturity, Total Estimated Fair Value 413  
Held-to-maturity Securities, Gross Unrealized Losses 2 85
Available for Sale, Less than 12 Months, Estimated Fair Value 49,657  
Available for Sale, Less than 12 Months, Unrealized Losses 290  
Available for Sale, More than 12 Months, Estimated Fair Value 44,458  
Available for Sale, More than 12 Months, Unrealized Losses 490  
Available for Sale, Total Estimated Fair Value 94,115  
Available-for-sale Securities, Gross Unrealized Loss, before Tax 780 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 41,619  
Available for Sale, Less than 12 Months, Unrealized Losses 413  
Available for Sale, More than 12 Months, Estimated Fair Value 0  
Available for Sale, More than 12 Months, Unrealized Losses 0  
Available for Sale, Total Estimated Fair Value 41,619  
Available-for-sale Securities, Gross Unrealized Loss, before Tax 413 72,690
Other Investments [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value 1,499  
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1  
Held to Maturity, More than 12 Months, Estimated Fair Value 0  
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0  
Held to Maturity, Total Estimated Fair Value 1,499  
Held-to-maturity Securities, Gross Unrealized Losses 1 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 1,722,158  
Available for Sale, More than 12 Months, Unrealized Losses 1,731  
Available for Sale, Total Estimated Fair Value 1,722,158  
Available-for-sale Securities, Gross Unrealized Loss, before Tax $ 1,731 $ 29,500