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Investment Securities Held to Maturity - Disclosure Regarding Length of Time on Investment Securities with Temporary Impairment (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]    
Less than 12 months, fair value $ 50,800 $ 180,374
12 months or longer, Fair value 49,488 6,617
Total, fair value 100,288 186,991
Less than 12 months, gross unrealized losses (465) (5,267)
12 months or longer, gross unrealized losses (1,443) (304)
Total, gross unrealized losses (1,908) (5,571)
Agency Obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Less than 12 months, fair value 3,821 3,525
12 months or longer, Fair value 0 0
Total, fair value 3,821 3,525
Less than 12 months, gross unrealized losses (87) (83)
12 months or longer, gross unrealized losses 0 0
Total, gross unrealized losses (87) (83)
State and municipal obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Less than 12 months, fair value 37,317 172,152
12 months or longer, Fair value 49,488 6,617
Total, fair value 86,805 178,769
Less than 12 months, gross unrealized losses (295) (5,132)
12 months or longer, gross unrealized losses (1,443) (304)
Total, gross unrealized losses (1,738) (5,436)
Corporate Obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Less than 12 months, fair value 9,662 4,697
12 months or longer, Fair value 0 0
Total, fair value 9,662 4,697
Less than 12 months, gross unrealized losses (83) (52)
12 months or longer, gross unrealized losses
Total, gross unrealized losses $ (83) $ (52)