Receivables and Allowance for Doubtful Receivables (Receivables by Type) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total receivables	$ 188,580	$ 184,296
Trade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total receivables	187,000	182,993
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total receivables	$ 1,580	$ 1,303