Annual report pursuant to Section 13 and 15(d)

Accumulated Other Comprehensive Income (Loss) - Additional Information (Detail)

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Accumulated Other Comprehensive Income (Loss) - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accumulated Other Comprehensive Income [Line Items]      
Net gain on sales of securities $ 1,748 $ 46 $ 6,611
Net unrealized gains (losses) recognized in earnings resulting from the redemption of investment securities (1,300) 314 (1,900)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]      
Accumulated Other Comprehensive Income [Line Items]      
Net gain on sales of securities $ 1,700 $ 46 $ 6,600