Consolidated Balance Sheets [Parenthetical] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Balance Sheets Parenthetical [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 3,534	$ 2,622
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	64,075,000	63,391,000
Common stock, shares outstanding	54,055,000	54,088,000
Treasury stock at cost, shares	10,200,000	9,303,000