Postretirement Benefits Other than Pensions - Change in Benefit Obligation (Details) - Postretirement Health Coverage [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	$ 2,735	$ 2,774
Interest cost	69	101	$ 96
Actuarial loss	144	196
Plan amendment	(763)
Benefits paid	(310)	(336)
Benefit obligation at end of year	$ 1,875	$ 2,735	$ 2,774