Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Provisions for doubtful accounts	$ 1,474	$ 2,009	$ 1,304
Allowance for Doubtful Accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	5,700	5,200	5,500
Provisions for doubtful accounts	1,474	2,009	1,304
Deductions - returns and write-offs	374	1,509	1,604
Balance at end of year	$ 6,800	$ 5,700	$ 5,200