Allowance for Loan and Lease Losses and Reserve for Unfunded Commitments - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Loans And Leases Receivable Disclosure [Line Items]
Increase (decrease) in allowance for loan and lease losses	$ 3,700,000	$ 2,000,000	$ 6,700,000	$ 5,100,000
Commitments outstanding on troubled debt restructurings	500,000		500,000		$ 0
Recorded investment in troubled debt restructurings that subsequently defaulted within twelve months	0	0	340,000	0
Reserve for unfunded commitments	1,400,000		1,400,000		923,000
Provisions for (credits to) unfunded commitments	411,000	299,000	496,000	272,000
Charge-offs or recoveries related to reserve for unfunded commitments			0		$ 0
Acquired Impaired Loans
Loans And Leases Receivable Disclosure [Line Items]
Increase (decrease) in allowance for loan and lease losses	$ 244,000	$ 559,000	$ (879,000)	$ 2,200,000