Allowance for Loan Losses - Additional Information (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans considered nonperforming (in days)				90 days
Threshold for evaluation for impairment of substandard and nonaccrual commercial loans	$ 500,000.0			$ 500,000.0
Change-off arising from participation loan agreement	1,234,000		$ 2,660,000	11,311,000	$ 11,423,000
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Change-off arising from participation loan agreement	141,000		37,000	373,000	2,100,000
Other consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Change-off arising from participation loan agreement	425,000		$ 425,000	1,298,000	$ 1,228,000
Other consumer | Vehicles and Equipment Liens
Financing Receivable, Allowance for Credit Losses [Line Items]
Change-off arising from participation loan agreement		$ 5,200,000
Participating loans, total exposure		4,900,000
Participating loans, direct exposure		$ 950,000
Participating loans, recoveries	$ 100,000
Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Increase in substandard loans resulted in loans downgraded				$ 87,800,000