Derivatives and Hedging Activities - Summary of Fair Values of Derivative Instruments on Balance Sheets (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Notional or Contractual Amount	$ 305,600,000	$ 298,200,000
Asset Derivatives	11,191,000	7,400,000
Liability Derivatives	10,934,000	7,368,000
Interest Rate Swaps [Member] | Loan Swaps [Member]
Derivatives, Fair Value [Line Items]
Notional or Contractual Amount	305,642,000	298,223,000
Asset Derivatives	11,048,000	7,351,000
Liability Derivatives	10,832,000	7,345,000
Interest Rate Loan Commitments [Member] | Other Contract [Member]
Derivatives, Fair Value [Line Items]
Notional or Contractual Amount	29,505,000	20,319,000
Asset Derivatives	143,000	49,000
Forward TBA Contracts [Member] | Other Contract [Member]
Derivatives, Fair Value [Line Items]
Notional or Contractual Amount	31,000,000	31,750,000
Liability Derivatives	$ 102,000	$ 23,000