Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
RAP
Net period benefit cost for the pension and postretirement plans
Service cost	$ 2,075	$ 2,165
Interest cost	1,623	2,008
Expected return on plan assets	(6,430)	(6,405)
Amortization of prior service cost	(18)	(19)
Amortization of actuarial loss (gain)	1,050	808
Total net periodic pension cost	(1,701)	(1,444)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Interest cost	13	20
Amortization of prior service cost	(19)	(19)
Total net periodic pension cost	$ (6)	$ 1