Commitments - Summary of Financial Instrument Commitments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fixed
Financial instrument commitments
Financial instrument commitments	$ 418	$ 315
Loan commitments	82,942	68,996
Fixed | Borrower
Financial instrument commitments
Financial instrument commitments	418	220
Fixed | Borrower | Financial standby
Financial instrument commitments
Financial instrument commitments	177	137
Fixed | Borrower | Performance standby
Financial instrument commitments
Financial instrument commitments	241	83
Fixed | Nonborrower | Performance standby
Financial instrument commitments
Financial instrument commitments		95
Variable
Financial instrument commitments
Financial instrument commitments	11,860	13,196
Loan commitments	260,061	270,989
Variable | Borrower
Financial instrument commitments
Financial instrument commitments	11,718	12,672
Variable | Borrower | Financial standby
Financial instrument commitments
Financial instrument commitments	3,770	4,047
Variable | Borrower | Commercial standby
Financial instrument commitments
Financial instrument commitments	354	126
Variable | Borrower | Performance standby
Financial instrument commitments
Financial instrument commitments	7,594	8,499
Variable | Nonborrower | Performance standby
Financial instrument commitments
Financial instrument commitments	142	524
Total.
Financial instrument commitments
Financial instrument commitments	12,278	13,511
Loan commitments	343,003	339,985
Total. | Borrower
Financial instrument commitments
Financial instrument commitments	12,136	12,892
Total. | Borrower | Financial standby
Financial instrument commitments
Financial instrument commitments	3,947	4,184
Total. | Borrower | Commercial standby
Financial instrument commitments
Financial instrument commitments	354	126
Total. | Borrower | Performance standby
Financial instrument commitments
Financial instrument commitments	7,835	8,582
Total. | Nonborrower | Performance standby
Financial instrument commitments
Financial instrument commitments	$ 142	$ 619