Pension and Postretirement Benefits Other Than Pension Plans - Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Plan [Member] | Qualified Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	$ 675	$ 878	$ 1,351	$ 1,756
Expected return on plan assets	(1,054)	(943)	(2,109)	(1,887)
Amortization of net actuarial loss	281	286	563	573
Total net periodic benefit cost (credit)	(98)	221	(195)	442
Postretirement Health Coverage [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	18	25	35	50
Amortization of net actuarial loss	64	62	127	123
Amortization of prior service credit	(329)	(329)	(657)	(657)
Total net periodic benefit cost (credit)	$ (247)	$ (242)	$ (495)	$ (484)