Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
RAP
Net period benefit cost for the pension and postretirement plans
Service cost	$ 2,165	$ 1,925
Interest cost	2,008	2,413
Expected return on plan assets	(6,405)	(6,075)
Amortization of prior service cost	(19)	(19)
Amortization of actuarial loss (gain)	808	65
Total net periodic pension cost	(1,444)	(1,691)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Interest cost	20	26
Amortization of prior service cost	(19)	(19)
Amortization of actuarial loss (gain)	0	(1)
Total net periodic pension cost	$ 1	$ 6