Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
RAP
Net period benefit cost for the pension and postretirement plans
Service cost	$ 1,893	$ 1,781
Interest cost	1,660	1,756
Expected return on plan assets	(4,755)	(4,881)
Amortization of prior service cost	(19)	(19)
Amortization of actuarial loss	463	488
Total net pension and periodic benefit cost	(759)	(875)
Other Pension Plan [Member]
Net period benefit cost for the pension and postretirement plans
Interest cost	433
Expected return on plan assets	(532)
Total net pension and periodic benefit cost	(99)
Postretirement Plan (b)
Net period benefit cost for the pension and postretirement plans
Interest cost	26	24
Amortization of prior service cost	(19)	(19)
Amortization of actuarial loss	2	0
Total net pension and periodic benefit cost	$ 9	$ 5