Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
CURRENT ASSETS:
Allowance for accounts receivable	$ 426	$ 472
Preferred stock
Preferred Stock, Par or Stated Value Per Share	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized	10,000,000	10,000,000
Shares issued (in shares)	0	0
Shares outstanding (in shares)	0	0
Class A
Common stock
Common Stock, Par or Stated Value Per Share	$ 0.001	$ 0.001
Common Stock, Shares Authorized	750,000,000	750,000,000
Common Stock, Shares, Issued	55,218,000	35,938,000
Common Stock, Shares, Outstanding	55,218,000	35,938,000
Class B Common Stock
Common stock
Common Stock, Par or Stated Value Per Share	$ 0.001	$ 0.001
Common Stock, Shares Authorized	300,000,000	300,000,000
Common Stock, Shares, Issued	148,482,000	173,624,000
Common Stock, Shares, Outstanding	148,482,000	173,624,000
Class C
Common stock
Common Stock, Par or Stated Value Per Share	$ 0.001	$ 0.001
Common Stock, Shares Authorized	75,000,000	75,000,000
Common Stock, Shares, Issued	42,945,000	42,945,000
Common Stock, Shares, Outstanding	42,945,000	42,945,000