Receivables and Allowance for Doubtful Receivables (Allowance for Doubtful Receivables Rollforward) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of year	$ 3,630	$ 4,542	$ 2,346
Charged to costs and expenses	8,516	6,359	3,994
Charged to other accounts	0	0	0
Write-offs net of recoveries	7,739	7,271	1,798
Balance at end of year	$ 4,407	$ 3,630	$ 4,542