Stockholders' Equity Accumulated Other comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ (4,878)	$ (5,840)
Other Comprehensive Income (Loss), before Reclassifications, before Tax	440	508
Amounts reclassified from accumulated other comprehensive income/(loss)	166	454
Net current-period other comprehensive income/(loss)	606	962
Accumulated other comprehensive loss at end of period	(4,272)	(4,878)
UnrealizedGainsLossesFromDefinedBenefitPensionAndPostretirementPlanItems [Member]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(5,360)	(5,580)
Other Comprehensive Income (Loss), before Reclassifications, before Tax	281	(254)
Amounts reclassified from accumulated other comprehensive income/(loss)	336	474
Net current-period other comprehensive income/(loss)	617	220
Accumulated other comprehensive loss at end of period	(4,743)	(5,360)
Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	482	(260)
Other Comprehensive Income (Loss), before Reclassifications, before Tax	159	762
Amounts reclassified from accumulated other comprehensive income/(loss)	(170)	(20)
Net current-period other comprehensive income/(loss)	(11)	742
Accumulated other comprehensive loss at end of period	$ 471	$ 482