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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
RAP
Net period benefit cost for the pension and postretirement plans
Service cost	$ 2,165	$ 1,925	$ 4,330	$ 3,850
Interest cost	2,008	2,413	4,015	4,825
Expected return on plan assets	(6,405)	(6,075)	(12,810)	(12,150)
Amortization of prior service cost	(19)	(19)	(38)	(38)
Amortization of actuarial loss (gain)	803	65	1,610	130
Total net periodic pension cost	(1,449)	(1,691)	(2,893)	(3,383)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Interest cost	20	26	39	52
Amortization of prior service cost	(19)	(19)	(38)	(38)
Amortization of actuarial loss (gain)	0	(1)	0	(2)
Total net periodic pension cost	$ 1	$ 6	$ 2	$ 13