Commitments and Contingencies (Narrative) (Details) $ in Thousands				3 Months Ended	6 Months Ended
	Apr. 10, 2018 USD ($)	Sep. 25, 2015 USD ($)	May 23, 2012 USD ($)	Mar. 31, 2018 Agreement	Jun. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Loss contingency accrual, payments			$ 1,750
Loss contingency, damages sought, value		$ 9,800
Number of tolling agreement with EPA and DOT | Agreement				6
Litigation settlement date					April 10, 2018
Litigation final settlement amount	$ 6,000