Pension and Postretirement Benefits Other Than Pension Plans - Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plan [Member] | Qualified Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	$ 676	$ 878
Expected return on plan assets	(1,055)	(944)
Amortization of net actuarial loss	282	287
Total net periodic benefit cost (credit)	(97)	221
Postretirement Health Coverage [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	17	25
Amortization of net actuarial loss	63	61
Amortization of prior service credit	(328)	(328)
Total net periodic benefit cost (credit)	$ (248)	$ (242)