Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income	$ 58,124	$ 44,675	$ 41,140
Employee Benefits, net of tax:
Amortization of prior service cost, net of tax of $(31), $(29) and $(27), respectively	(46)	(48)	(40)
Net gain, net of tax of $432, $178, and $73, respectively	663	268	103
Cash Flow Hedges, net of tax:
Unrealized (loss)/gain on commodity contract cash flow hedges, net of tax of $(8), $496 and $(150), respectively	(11)	742	(227)
Total Other Comprehensive Income (Loss)	606	962	(164)
Comprehensive Income	$ 58,730	$ 45,637	$ 40,976