Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Plan:
Net period benefit cost for the pension and postretirement plans
Service cost	$ 1,781	$ 1,725
Interest cost	1,756	1,780
Expected return on plan assets	(4,881)	(5,065)
Amortization of prior service cost	(19)	13
Amortization of actuarial loss	488	482
Total net pension cost	(875)	(1,065)
Postretirement Plan:
Net period benefit cost for the pension and postretirement plans
Interest cost	24	36
Amortization of prior service cost	(19)	0
Total net pension cost	$ 5	$ 36