Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
RAP
Net period benefit cost for the pension and postretirement plans
Service cost	$ 2,075	$ 2,165	$ 4,151	$ 4,330
Interest cost	1,623	2,008	3,245	4,015
Expected return on plan assets	(6,430)	(6,405)	(12,861)	(12,810)
Amortization of prior service cost	(18)	(19)	(37)	(38)
Amortization of actuarial loss (gain)	1,050	803	2,100	1,610
Total net periodic pension cost	(1,701)	(1,449)	(3,402)	(2,893)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Interest cost	13	20	26	39
Amortization of prior service cost	(19)	(19)	(38)	(38)
Total net periodic pension cost	$ (6)	$ 1	$ (12)	$ 2