Employee Benefit and Deferred Compensation Plans (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation
Benefit obligation at beginning of year	$ 28,012,000
Benefit obligation at end of year	27,859,000	$ 28,012,000
Change in fair value of plan assets
Fair value of plan assets at beginning of year	25,241,000
Fair value of plan assets at end of year	26,913,000	25,241,000
Pension Benefits
Change in benefit obligation
Benefit obligation at beginning of year	28,012,000	27,856,000
Service cost	0	0	$ 0
Interest cost	1,168,000	1,216,000	1,095,000
Plan participants’ contributions	0	0
Actuarial loss (gain)	582,000	(115,000)
Benefits paid	(1,903,000)	(2,018,000)
Settlements	0	0
Transfer from HeritageBank Pension Plan	0	1,073,000
Benefit obligation at end of year	27,859,000	28,012,000	27,856,000
Change in fair value of plan assets
Fair value of plan assets at beginning of year	25,241,000	24,434,000
Actual return on plan assets	3,575,000	1,752,000
Contribution by employer	0	0
Benefits paid	(1,903,000)	(2,018,000)
Settlements	0	0
Expenses paid from plan trust	0	0
Transfer from HeritageBank Pension Plan	0	1,073,000
Fair value of plan assets at end of year	26,913,000	25,241,000	24,434,000
Funded status at end of year	$ (946,000)	$ (2,771,000)
Weighted-average assumptions as of December 31
Discount rate (percent)	3.96%	4.35%
Pension Benefits | HeritageBank
Change in benefit obligation
Benefit obligation at beginning of year	$ 0	$ 12,913,000
Service cost		0	0
Interest cost		172,000	305,000
Plan participants’ contributions		0
Actuarial loss (gain)		(481,000)
Benefits paid		(22,000)
Settlements		(11,509,000)
Transfer from HeritageBank Pension Plan		(1,073,000)
Benefit obligation at end of year		0	12,913,000
Change in fair value of plan assets
Fair value of plan assets at beginning of year	0	12,458,000
Actual return on plan assets		29,000
Contribution by employer		142,000
Benefits paid		(22,000)
Settlements		(11,509,000)
Expenses paid from plan trust		(25,000)
Transfer from HeritageBank Pension Plan		(1,073,000)
Fair value of plan assets at end of year		0	12,458,000
Funded status at end of year		$ 0
Weighted-average assumptions as of December 31
Discount rate (percent)		0.00%
Other Benefits
Change in benefit obligation
Benefit obligation at beginning of year	1,566,000	$ 1,704,000
Service cost	9,000	12,000	17,000
Interest cost	42,000	58,000	60,000
Plan participants’ contributions	77,000	76,000
Actuarial loss (gain)	(328,000)	(56,000)
Benefits paid	(196,000)	(228,000)
Settlements	0	0
Transfer from HeritageBank Pension Plan	0	0
Benefit obligation at end of year	1,170,000	1,566,000	$ 1,704,000
Change in fair value of plan assets
Settlements	0	0
Funded status at end of year	$ (1,170,000)	$ (1,566,000)
Weighted-average assumptions as of December 31
Discount rate (percent)	3.37%	3.57%