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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
RAP
Net period benefit cost for the pension and postretirement plans
Service cost	$ 1,893	$ 1,782	$ 3,785	$ 3,563
Interest cost	1,660	1,756	3,320	3,512
Expected return on plan assets	(4,755)	(4,882)	(9,510)	(9,763)
Amortization of prior service cost	(19)	(18)	(38)	(37)
Amortization of actuarial loss	463	487	925	975
Total net pension and periodic benefit cost	(759)	(875)	(1,518)	(1,750)
Other Pension Plan [Member]
Net period benefit cost for the pension and postretirement plans
Interest cost	650		1,083
Expected return on plan assets	(1,060)		(1,592)
Total net pension and periodic benefit cost	(410)		(509)
Postretirement Plan(b)
Net period benefit cost for the pension and postretirement plans
Interest cost	27	24	53	48
Amortization of prior service cost	(19)	(19)	(38)	(38)
Amortization of actuarial loss	2	0	4	0
Total net pension and periodic benefit cost	$ 10	$ 5	$ 19	$ 10