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Securities - Gross Unrealized Losses and Fair Value of Company's Investments (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months $ 389,696 $ 425,635
Fair value more than 12 months 9,626 10,199
Total fair value 399,323 435,834
Unrealized losses less than 12 months (8,110) (9,413)
Unrealized losses more than 12 months (232) (252)
Total unrealized losses (8,342) (9,665)
U.S. Treasury and Federal Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months 13,158 6,987
Total fair value 13,158 6,987
Unrealized losses less than 12 months (68) (64)
Total unrealized losses (68) (64)
State and Municipal [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months 107,096 142,466
Total fair value 107,096 142,466
Unrealized losses less than 12 months (2,607) (3,544)
Total unrealized losses (2,607) (3,544)
Federal Agency Collateralized Mortgage Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months 105,289 112,414
Fair value more than 12 months 9,626 10,199
Total fair value 114,916 122,613
Unrealized losses less than 12 months (1,700) (1,918)
Unrealized losses more than 12 months (232) (252)
Total unrealized losses (1,932) (2,170)
Federal Agency Mortgage-backed Pools [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months 162,271 163,768
Total fair value 162,271 163,768
Unrealized losses less than 12 months (3,716) (3,887)
Total unrealized losses (3,716) $ (3,887)
Private Labeled Mortgage-backed Pools [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value less than 12 months 1,882  
Total fair value 1,882  
Unrealized losses less than 12 months (19)  
Total unrealized losses $ (19)