Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Comprehensive Income (Loss) (AOCI) by component, net of tax
Beginning balance, January 1	$ 21,884
Ending balance, December 31,	13,807	$ 21,884
Post retirement medical plan | Accumulated Other Comprehensive Income (Loss)
Other Comprehensive Income (Loss) (AOCI) by component, net of tax
Beginning balance, January 1	21,884	(12,951)	$ (11,299)
Other comprehensive income (loss) before reclassifications	(3,489)	38,144	(2,441)
Amounts reclassified from accumulated other comprehensive income (loss)	(4,588)	(3,309)	789
Net current period other comprehensive income (loss)	(8,077)	34,835	(1,652)
Ending balance, December 31,	$ 13,807	$ 21,884	$ (12,951)