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Investment Securities (Held to Maturity) (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost $ 481,845 $ 488,183
Gross unrealized gains 10,430 6,675
Gross unrealized losses (1,850) (5,571)
Fair value 490,425 489,287
Agency Obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 4,307 4,306
Gross unrealized gains 0 2
Gross unrealized losses (55) (83)
Fair value 4,252 4,225
Mortgage-Backed Securities [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 475 893
Gross unrealized gains 17 31
Gross unrealized losses 0 0
Fair value 492 924
State And Municipal Obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 467,113 473,653
Gross unrealized gains 10,407 6,635
Gross unrealized losses (1,761) (5,436)
Fair value 475,759 474,852
Corporate Obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 9,950 9,331
Gross unrealized gains 6 7
Gross unrealized losses (34) (52)
Fair value $ 9,922 $ 9,286