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Securities - Gross Unrealized Losses and Fair Value of Company's Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months $ 425,635 $ 225,524
Fair value more than 12 months 10,199 42,380
Total fair value 435,834 267,904
Unrealized losses less than 12 months (9,413) (2,633)
Unrealized losses more than 12 months (252) (980)
Total unrealized losses (9,665) (3,613)
U.S. Treasury and Federal Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months 6,987 5,468
Total fair value 6,987 5,468
Unrealized losses less than 12 months (64) (17)
Total unrealized losses (64) (17)
State and Municipal [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months 142,466 17,353
Fair value more than 12 months   446
Total fair value 142,466 17,799
Unrealized losses less than 12 months (3,544) (280)
Unrealized losses more than 12 months   (6)
Total unrealized losses (3,544) (286)
Federal Agency Collateralized Mortgage Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months 112,414 89,459
Fair value more than 12 months 10,199 25,428
Total fair value 122,613 114,887
Unrealized losses less than 12 months (1,918) (1,124)
Unrealized losses more than 12 months (252) (655)
Total unrealized losses (2,170) (1,779)
Federal Agency Mortgage-backed Pools [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months 163,768 113,244
Fair value more than 12 months   16,506
Total fair value 163,768 129,750
Unrealized losses less than 12 months (3,887) (1,212)
Unrealized losses more than 12 months   (319)
Total unrealized losses $ (3,887) $ (1,531)