Accounts Receivable - Financing Receivables (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivables	$ 34,268	$ 28,683
Sales-type lease receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivables	22,147	16,558
Trade receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivables	12,121	12,125
Scheduled to be received in excess of one year
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivables	$ 18,352	$ 15,191