Postretirement Benefits Other than Pensions - Net Periodic Postretirement Benefit (Income) Expense (Details) - Postretirement Health Coverage [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	$ 96	$ 108	$ 120
Amortization of net actuarial loss	257	245	242
Amortization of prior service credit	(1,314)	(1,314)	(1,314)
Total net periodic benefit cost (credit)	$ (961)	$ (961)	$ (952)