Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Customer receivables, allowances	$ 9,743	$ 9,465
Common stock, par value	$ 0.13	$ 0.10
Common stock, shares authorized	140,000,000	140,000,000
Common stock, shares outstanding	82,284,180	82,375,945