Receivables (Schedule of Allowance for Doubtful Accounts) (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 period	$ 6,505	$ 1,446	$ 2,973
Charged to costs and expenses	2,001	6,463	1,242
Amounts written off	(38)	(1,404)	(2,769)
Balance at end of period	$ 8,468	$ 6,505	$ 1,446