Postretirement Benefits (Details 8) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Amounts recognized in other comprehensive income for pension plans:
Net actuarial loss (gain) arising during the year	$ 44,524	$ (23,810)
Amortization of net actuarial loss	(8,111)	(14,031)
Amortization of prior service cost	(696)	(670)
Amendments	2,114	0
Total recognized in other comprehensive income	$ 37,831	$ (38,511)
Number of new plan participants resulting in charges recognized as amendments	1
Employee pension plan
Amounts recognized in other comprehensive income for pension plans:
Amendments	$ 1,700
Supplemental employee retirement plan (SERP)
Amounts recognized in other comprehensive income for pension plans:
Amendments	$ 400