Retirement Plans, Components of Net Pension Cost for the Retirement Account Plan Tables (Details) - RAP - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net period benefit cost for the pension and postretirement plans
Service cost	$ 8,244	$ 7,263	$ 7,540
Interest cost	8,185	9,752	9,125
Expected return on plan assets	(25,595)	(24,332)	(23,195)
Amortization of prior service cost	(73)	(73)	(73)
Amortization of actuarial loss (gain)	3,897	480	2,195
Recognized settlement loss (gain)	0	0	809
Total net periodic pension cost (income)	$ (5,342)	$ (6,910)	$ (3,600)