CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts	$ 37,912	$ 44,730
Preferred stock, par value (in USD per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in USD per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	250,000,000	125,000,000
Common stock, shares issued (in shares)	127,634,012	117,682,543
Common stock, shares outstanding (in shares)	127,634,012	117,682,543
Treasury stock (in shares)	5,106	0
Series A Preferred Stock
Convertible preferred stock series, par value (in USD per share)	$ 0.0001	$ 0.0001
Convertible preferred stock series, shares authorized (in shares)	825,000	825,000
Convertible preferred stock series, shares issued (in shares)	21,645	21,645
Convertible preferred stock series, shares outstanding (in shares)	21,645	21,645
Convertible preferred stock series, liquidation preference	$ 2,916	$ 2,603
Series C Preferred Stock
Convertible preferred stock series, par value (in USD per share)	$ 0.0001	$ 0.0001
Convertible preferred stock series, shares authorized (in shares)	625,000	625,000
Convertible preferred stock series, shares issued (in shares)	614,177	614,177
Convertible preferred stock series, shares outstanding (in shares)	614,177	614,177
Convertible preferred stock series, liquidation preference	$ 84,555	$ 75,491