Pension and Postretirement Benefits Other Than Pension Plans - Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Plan [Member] | Qualified Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	$ 534	$ 798	$ 1,885	$ 2,554
Expected return on plan assets	(962)	(935)	(3,071)	(2,822)
Amortization of net actuarial loss	340	293	903	866
Net settlement loss	1,343	1,577	1,343	1,577
Total net periodic benefit expense (income)	1,255	1,733	1,060	2,175
Postretirement Health Coverage [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	17	26	52	76
Amortization of net actuarial loss	57	55	184	178
Amortization of prior service credit	(329)	(329)	(986)	(986)
Total net periodic benefit expense (income)	$ (255)	$ (248)	$ (750)	$ (732)