Postretirement Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service cost	$ 39	$ 52	$ 53
Unrecognized actuarial losses	551	392	593
Total loss recognized in Accumulated Other Comprehensive Income	590	444	646
Income tax effect	(206)	(155)	(226)
Net loss recognized in Accumulated Other Comprehensive Income	$ 384	$ 289	$ 420