Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Account Plan
Net period benefit cost for the pension and postretirement plans
Service cost	$ 6,955	$ 6,780	$ 11,257
Interest cost	7,121	7,121	6,617
Expected return on plan assets	(19,646)	(20,287)	(21,438)
Prior service cost	(73)	(73)	50
Actuarial (gain) loss	2,278	2,115	2,256
Total net pension cost	(3,365)	(4,344)	(1,258)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Service cost	0	0	0
Interest cost	101	142	141
Prior service cost	(75)	0	0
Actuarial (gain) loss	4	0	0
Total net pension cost	$ 30	$ 142	$ 141