Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Defined postretirement benefit obligation	$ 8,385	$ 11,476
Allowance for loan losses	5,671	7,886
Accrued expenses	910	1,787
Federal net operating loss carryforwards	650	1,740
Stock compensation	1,272	1,698
State income taxes	975	1,515
Premises and equipment	649	990
Securities available-for-sale	400	598
California net operating loss carryforwards	394	454
Split-dollar life insurance benefit plan	76	102
Tax credit carryforwards	70	101
Nonaccrual interest	76
Other	365	1,069
Total deferred tax assets	19,893	29,416
Deferred tax liabilities:
Intangible assets	(1,357)	(1,614)
Loan fees	(1,079)	(1,278)
Prepaid expenses	(533)	(644)
I/O strips	(406)	(448)
FHLB stock	(171)	(244)
Other	(100)	(130)
Total deferred tax liabilities	(3,646)	(4,358)
Net deferred tax assets	$ 16,247	$ 25,058