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Intangible Assets - Amortization Expense (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]      
Core deposit premiums $ 1,076 $ 1,300 $ 1,757
Customer relationship and other intangibles 1,474 1,909 2,136
Mortgage servicing rights 120 182 173
Total amortization expense of intangible assets $ 2,670 $ 3,391 $ 4,066