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Investment Securities Held to Maturity (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost $ 477,652 $ 488,183
Gross unrealized gains 9,295 6,675
Gross unrealized losses (1,908) (5,571)
Fair value 485,039 489,287
Agency Obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 4,308 4,306
Gross unrealized gains 0 2
Gross unrealized losses (87) (83)
Fair value 4,221 4,225
Mortgage-backed securities [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 382 893
Gross unrealized gains 14 31
Gross unrealized losses 0 0
Fair value 396 924
State and municipal obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 462,942 473,653
Gross unrealized gains 9,280 6,635
Gross unrealized losses (1,738) (5,436)
Fair value 470,484 474,852
Corporate Obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 10,020 9,331
Gross unrealized gains 1 7
Gross unrealized losses (83) (52)
Fair value $ 9,938 $ 9,286