Borrowed Funds - Schedule of Repayment of Long-Term Borrowings for Securities Sold Under Agreements to Repurchase (Detail) - Long-term borrowings - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
2018	$ 50,000
2022	50,000
Total long-term securities sold under agreements to repurchase	$ 100,000	$ 165,000