CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' (DEFICIT) EQUITY - USD ($) $ in Thousands	Total	Convertible Preferred Stock [Member]	Common Stock Class A [Member]	Common Stock Class B [Member]	Common Stock Class C [Member]	Common Stock Class D [Member]	Additional Paid-In Capital	Accumulated Deficit [Member]
BALANCE at Dec. 31, 2016	$ (71,126)	$ 0	$ 2	$ 3	$ 3	$ 41	$ 981,688	$ (1,052,863)
Consolidated net income	111,887	0	0	0	0	0	0	111,887
Stock-based compensation expense	4,647	0	0	0	0	2	4,645	0
Conversion of 51,467 shares of Class A common stock to Class D common stock	0	0	0	0	0	0	0	0
Repurchase shares of Class A common stock and Class D common stock	(4,958)	0	0	0	0	(2)	(4,956)	0
Repurchase of share-based equity awards	0
Adjustment of redeemable noncontrolling interests to estimated redemption value	2,205	0	0	0	0	0	2,205	0
BALANCE at Dec. 31, 2017	42,655	0	2	3	3	41	983,582	(940,976)
Consolidated net income	141,001	0	0	0	0	0	0	141,001
Stock-based compensation expense	4,711	0	0	0	0	2	4,709	0
Repurchase shares of Class A common stock and Class D common stock	(8,168)	0	0	0	0	(4)	(8,164)	0
Repurchase of share-based equity awards	(1,077)	0	0	0	0	0	(1,077)	0
Exercise of options for 63,190 shares of common stock	94	0	0	0	0	0	94	0
Adjustment of redeemable noncontrolling interests to estimated redemption value	(516)	0	0	0	0	0	(516)	0
BALANCE at Dec. 31, 2018	$ 178,700	$ 0	$ 2	$ 3	$ 3	$ 39	$ 978,628	$ (799,975)