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Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($)
$ in Thousands
Sep. 30, 2015
Dec. 31, 2014
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value $ 295,618  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 3,233  
Held to Maturity, More than 12 Months, Estimated Fair Value 211,796  
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 5,225  
Held to Maturity, Total, Estimated Fair Value 507,414  
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss 8,458 $ 9,035
Available for Sale, Less than 12 Months, Estimated Fair Value 812,212  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 9,256  
Available for Sale, More than 12 Months, Estimated Fair Value 11,326  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 318  
Available for Sale, Total, Estimated Fair Value 823,538  
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 9,574  
U.S. Treasury [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss 0 0
Residential mortgage-backed securities [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss 0 0
Available for Sale, Less than 12 Months, Estimated Fair Value 25,945  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 148  
Available for Sale, More than 12 Months, Estimated Fair Value 11,326  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 318  
Available for Sale, Total, Estimated Fair Value 37,271  
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 466  
States and political subdivisions [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value 295,618  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 3,233  
Held to Maturity, More than 12 Months, Estimated Fair Value 211,796  
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 5,225  
Held to Maturity, Total, Estimated Fair Value 507,414  
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss 8,458 $ 9,035
Available for Sale, Less than 12 Months, Estimated Fair Value 786,267  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 9,108  
Available for Sale, More than 12 Months, Estimated Fair Value 0  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0  
Available for Sale, Total, Estimated Fair Value 786,267  
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss $ 9,108