CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Common stock par value (in dollars per share)	$ 0.01	$ 0.01
Common stock shares authorized (in shares)	140,000,000	140,000,000
Common stock shares issued (in shares)	53,293,449	53,256,098
Common stock shares outstanding (in shares)	51,071,608	51,622,898
Preferred stock par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock shares authorized (in shares)	10,000,000	10,000,000
Preferred stock shares issued (in shares)	0	0
Preferred stock shares outstanding (in shares)	0	0
Treasury stock (in shares)	2,221,841,000	1,633,200,000