Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	240,000,000	240,000,000
Common stock, shares issued	69,770,795	73,549,872
Common stock, shares outstanding	68,196,964	69,326,202
Treasury stock, shares	1,573,831	4,223,670
Voting Common Stock
Common stock, par value (in dollars per share)	$ 0.0001
Common stock, shares authorized	240,000,000