CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Short-term Investments	$ 24,965	$ 15,048
Common stock, par value (in dollars per share)	$ 0.10	$ 0.10
Common stock, shares authorized (in shares)	280,000,000	280,000,000
Common stock, shares issued (in shares)	148,823,774	148,107,038
Common Stock, Shares, Outstanding	138,105,455	140,103,841
Treasury stock shares (in shares)	10,718,319	8,003,197
Tax effect of accumulated other comprehensive income	$ 0	$ 0