Postretirement Benefits (Details 8) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Amounts recognized in other comprehensive income for pension plans:
Net actuarial loss arising during the year	$ 15,588	$ 44,524
Amortization of net actuarial loss	(9,301)	(8,111)
Amortization of prior service cost	(871)	(696)
Amendments	5,050	2,114
Impact due to settlement	(302)
Total recognized in other comprehensive income	10,164	37,831
Impact of amendments on pension obligation	5,050	2,114
Employee pension plan | Qualified plan | Funded plan
Amounts recognized in other comprehensive income for pension plans:
Impact of amendments on pension obligation	3,600
Impact of amendments on prior service cost		1,700
Supplemental employee retirement plan (SERP) | Nonqualified plan | Unfunded plan
Amounts recognized in other comprehensive income for pension plans:
Impact of amendments on pension obligation	300
Impact of amendments on prior service cost	$ 1,200	$ 400
Number of new plan participants resulting in charges recognized as amendments	2	1
Number of former plan participants resulting in charges recognized as settlements	2