Employee Benefits Plans (Details 13) - Other Postretirement Benefits - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amounts recognized in the Consolidated Balance Sheets
Current liability	$ 1,481	$ 1,506
Noncurrent liability	19,760	19,459
Net amount recognized	21,241	20,965
Amounts recognized in accumulated other comprehensive loss (pre-tax)
Prior service cost	(154)	(385)
Net actuarial gain	(8,371)	(10,186)
Net amount recognized	$ (8,525)	$ (10,571)