Commitments and Contingencies (Narrative) (Details) $ in Thousands				3 Months Ended			12 Months Ended
	Feb. 28, 2018 USD ($)	Sep. 25, 2015 USD ($)	May 23, 2012 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Agreement	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Site Contingency [Line Items]
Operating leases expiration, year							2033
Rent expense							$ 4,500	$ 4,200	$ 4,600
Loss contingency accrual, payments			$ 1,750
Loss contingency, damages sought, value		$ 9,800
Litigation settlement, expense				$ 5,000	$ 800	$ 200
Number of tolling agreement with EPA and DOT | Agreement							6
Litigation settlement date							February 9, 2018
Litigation filed date							February 12, 2018
Subsequent Event [Member]
Site Contingency [Line Items]
Litigation settlement amount	$ 6,000