Derivative Instruments and Hedging Activities - Derivatives Not Designated as Hedging Instruments (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Notional Amount	$ 132,593	$ 107,448
Derivative Assets	588	1,058
Derivative Liabilities	74	74
Derivative instruments not designated as hedging instruments gain (loss), net	67	(164)	$ (311)
Interest Rate Swap [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Notional Amount	523	622
Interest Rate Swap [Member] | Other Assets [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative Assets	0	0
Interest Rate Swap [Member] | Other Liabilities [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative Liabilities	38	65
Credit Risk Contract [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Notional Amount	75,622	27,919
Credit Risk Contract [Member] | Other Noninterest Income [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative instruments not designated as hedging instruments gain (loss), net	403	93	0
Credit Risk Contract [Member] | Other Assets [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative Assets	0	0
Credit Risk Contract [Member] | Other Liabilities [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative Liabilities	36	9
Interest Rate Locks with Customers [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Notional Amount	27,411	36,541
Interest Rate Locks with Customers [Member] | Mortgage Banking Activities [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative instruments not designated as hedging instruments gain (loss), net	274	288	(301)
Interest Rate Locks with Customers [Member] | Other Assets [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative Assets	527	801
Interest Rate Locks with Customers [Member] | Other Liabilities [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative Liabilities	0	0
Forward Loan Sale Commitments [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Notional Amount	29,037	42,366
Forward Loan Sale Commitments [Member] | Mortgage Banking Activities [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative instruments not designated as hedging instruments gain (loss), net	196	(359)	$ (10)
Forward Loan Sale Commitments [Member] | Other Assets [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative Assets	61	257
Forward Loan Sale Commitments [Member] | Other Liabilities [Member]
Derivative Notional Amount And Fair Value By Balance Sheet Not Designated As Hedging Instrument [Line Items]
Derivative Liabilities	$ 0	$ 0