Description of Business and Summary of Significant Accounting Policies (Accounts Receivables and Allowances) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 117,443		$ 117,838
Allowance for doubtful accounts	5,138		$ 4,143
Deferred revenue included in accounts receivables	7,800
Provision for doubtful accounts	1,045	$ 407
Allowance for Doubtful Accounts Receivable, Write-offs	40	$ 82
Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	85,400
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 32,000