CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Stockholders' equity:
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	100,000,000	50,000,000
Common stock, shares issued (in shares)	47,370,945	45,895,086
Common stock, shares outstanding (in shares)	33,249,665	33,865,688
Treasury stock, shares (in shares)	14,121,280	12,029,398