Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Allowance for doubtful accounts	$ 8,229	$ 8,857
Equity:
Preferred stock, par value (in dollars per share)	$ 0.00001	$ 0.00001
Preferred stock, shares authorized (in shares)	500,000	500,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.00001	$ 0.00001
Common stock, shares authorized (in shares)	180,000,000	180,000,000
Common stock, shares issued (in shares)	80,885,000	84,368,000
Common stock, shares outstanding (in shares)	80,885,000	84,368,000