Postretirement Benefits Other than Pensions - Net Periodic Postretirement Benefit (Income) Expense (Details) - Postretirement Health Coverage [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	$ 69	$ 101	$ 96
Amortization of net actuarial loss	245	238	257
Amortization of prior service credit	(1,314)	(1,314)	(1,314)
Total net periodic benefit expense (income)	$ (1,000)	$ (975)	$ (961)