Pension and Postretirement Benefit Plans - Changes in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SERP I and SERP II
Change in benefit obligations
Benefit obligation at January 1	$ 5,158	$ 4,687
Service cost	0	0	$ 0
Interest cost	140	179	160
Plan participants contribution	0	0
Actuarial loss (gain)	439	595
Settlements	0	0
Benefits paid	(347)	(303)
Benefit obligation at December 31	5,390	5,158	4,687
Change in plan assets
Fair value of plan assets at January 1	0	0
Actual return on plan assets	0	0
Settlements	0	0
Excess assets transferred to defined contribution plan	0	0
Employer contribution	347	303
Plan participants’ contribution	0	0
Benefits paid	(347)	(303)
Fair value of plan assets at December 31	0	0	0
Funded status at year end (plan assets less benefit obligations)	(5,390)	(5,158)
Accrued liability	(2,998)	(3,112)
Net actuarial loss	(2,392)	(2,046)
Prior service cost	0	0
Unrecognized net initial obligation	0	0
PRBP
Change in benefit obligations
Benefit obligation at January 1	235	241
Service cost	0	0	0
Interest cost	5	8	9
Plan participants contribution	36	38
Actuarial loss (gain)	4	40
Settlements	0	0
Benefits paid	(80)	(92)
Benefit obligation at December 31	200	235	241
Change in plan assets
Fair value of plan assets at January 1	0	0
Actual return on plan assets	0	0
Settlements	0	0
Excess assets transferred to defined contribution plan	0	0
Employer contribution	44	54
Plan participants’ contribution	36	38
Benefits paid	(80)	(92)
Fair value of plan assets at December 31	0	0	$ 0
Funded status at year end (plan assets less benefit obligations)	(200)	(235)
Accrued liability	(84)	(99)
Net actuarial loss	(116)	(136)
Prior service cost	0	0
Unrecognized net initial obligation	$ 0	$ 0