Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plan:
Net period benefit cost for the pension and postretirement plans
Service cost	$ 1,782	$ 1,725	$ 3,563	$ 3,450
Interest cost	1,756	1,780	3,512	3,560
Expected return on plan assets	(4,882)	(5,065)	(9,763)	(10,130)
Amortization of prior service cost	(18)	12	(37)	25
Amortization of actuarial loss	487	483	975	965
Total net pension and periodic benefit cost	(875)	(1,065)	(1,750)	(2,130)
Postretirement Plan:
Net period benefit cost for the pension and postretirement plans
Interest cost	24	36	48	72
Amortization of prior service cost	(19)	0	(38)	0
Total net pension and periodic benefit cost	$ 5	$ 36	$ 10	$ 72