Receivables and Allowance for Doubtful Receivables (Allowance for Doubtful Receivables Rollforward) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of year	$ 4,407	$ 3,630	$ 4,542
Charged to costs and expenses	5,800	8,516	6,359
Charged to other accounts	0	0	0
Write-offs net of recoveries	6,215	7,739	7,271
Balance at end of year	$ 3,992	$ 4,407	$ 3,630