Schedule II - Valuation and Qualifying Accounts (Valuation and Qualifying Accounts) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Sales returns and allowances [Member]
Valuation and Qualifying Accounts
Balance at beginning of period		$ 9,595	$ 7,978	$ 6,855
Additions - charged to costs and expenses		1,347	1,617	1,123
Additions - charged to other accounts - describe		0	0	0
Deductions - describe		0	0	0
Balance at end of period		10,942	9,595	7,978
Allowance for doubtful accounts [Member]
Valuation and Qualifying Accounts
Balance at beginning of period		12,040	9,637	8,713
Additions - charged to costs and expenses		8,598	9,587	7,119
Additions - charged to other accounts - describe		0	0	0
Deductions - describe	[1]	7,921	7,184	6,195
Balance at end of period		$ 12,717	$ 12,040	$ 9,637

[1] Uncollectable accounts written off.