Unaudited Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Long-term debt, fair value	$ 55,878	$ 0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	260,000,000	260,000,000
Common stock, shares issued	107,765,000	90,159,000
Treasury stock, shares	32,934,000	32,820,000