Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Customer receivables, allowances	$ 10,215	$ 9,913
Common stock, par value	$ 0.10	$ 0.10
Common stock, shares authorized	140,000,000	140,000,000
Common stock, shares outstanding	81,071,908	81,231,131