Pension and Other Postretirement Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 30	$ 27
Interest cost	132	168
Expected return on plan assets	(356)	(315)
Amortization of net loss	134	145
Net periodic benefit cost	(60)	25
Chairman and President Non-Qualified Defined Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	309	302
Interest cost	261	338
Amortization of prior service cost	15	15
Amortization of net loss	1,282	1,445
Net periodic benefit cost	1,867	2,100
Defined Benefit Postretirement Healthcare Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	37	41
Amortization of net loss	73	40
Net periodic benefit cost	$ 110	$ 81