Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Retirement Account Plan
Net period benefit cost for the pension and postretirement plans
Service cost	$ 6,780	$ 11,257	$ 11,058
Interest cost	7,121	6,617	7,132
Expected return on plan assets	(20,287)	(21,438)	(19,922)
Prior service cost	(73)	50	58
Actuarial (gain) loss	2,115	2,256	1,384
Total net pension cost	(4,344)	(1,258)	(290)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Service cost	0	0	0
Interest cost	142	141	150
Prior service cost	0	0	0
Actuarial (gain) loss	0	0	(35)
Total net pension cost	$ 142	$ 141	$ 115