Retirement Plans, Components of Net Pension Cost for the Retirement Account Plan Tables (Details) - Retirement Account Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net period benefit cost for the pension and postretirement plans
Service cost	$ 7,540	$ 6,955	$ 6,780
Interest cost	9,125	7,121	7,121
Expected return on plan assets	(23,195)	(19,646)	(20,287)
Amortization of prior service cost	(73)	(73)	(73)
Amortization of actuarial loss (gain)	2,195	2,278	2,115
Recognized settlement loss (gain)	809	0	0
Total net periodic pension cost	$ (3,600)	$ (3,365)	$ (4,344)