Retirement Plans and Other Postretirement Benefits - Components of Net Periodic Cost, Changes in Plan Assets and Benefit Obligations, and AOCI Impacts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan
Service cost:
Benefit accrual	$ 0	$ 0	$ 0
Other expenses	650	650	650
Interest cost	731	997	947
Expected return on plan assets	(966)	(1,222)	(1,377)
Settlement loss	447	0	0
Amortization of:
Prior service cost	0	0	0
Actuarial loss	342	310	371
Net periodic pension expense	1,204	735	591
Changes in plan assets and benefit obligations included in other comprehensive income (loss):
Prior service cost	0	0	0
Net actuarial loss (gain)	277	(1,755)	177
Amortization of:
Prior service cost	0	0	0
Actuarial loss	(790)	(310)	(371)
Total recognized in other comprehensive income (loss)	$ (513)	$ (2,065)	$ (194)
Weighted average assumptions used to determine expense:
Discount rate	3.10%	4.20%	3.50%
Expected return on plan assets	4.80%	5.75%	5.90%
Weighted average assumptions used to determine benefit obligations as of December 31:
Discount rate	2.08%	3.10%	4.20%
Expected return on plan assets	4.80%	5.75%	5.90%
Supplemental Defined Benefit Plans
Service cost:
Benefit accrual	$ 0	$ 0	$ 0
Other expenses	0	0	0
Interest cost	453	620	566
Expected return on plan assets	0	0	0
Settlement loss	0	0	0
Amortization of:
Prior service cost	0	0	0
Actuarial loss	302	256	310
Net periodic pension expense	755	876	876
Changes in plan assets and benefit obligations included in other comprehensive income (loss):
Prior service cost	0	0	0
Net actuarial loss (gain)	1,318	516	(789)
Amortization of:
Prior service cost	0	0	0
Actuarial loss	(302)	(256)	(310)
Total recognized in other comprehensive income (loss)	$ 1,016	$ 260	$ (1,099)
Weighted average assumptions used to determine expense:
Discount rate	3.10%	4.20%	3.50%
Weighted average assumptions used to determine benefit obligations as of December 31:
Discount rate	2.08%	3.10%	4.20%