Condensed Consolidated Statement of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Additional Paid-in Capital	Retained Earnings (Deficit)	Non-controlling Interest	Class A common stock	Class B common stock	Class C common stock	Total
Increase (Decrease) in Members' Equity (Deficit)
Adoption of accounting standard (see Note 1 - Summary of Significant Accounting Policies) | ASU 2014-09		$ 1,310	$ 2,476				$ 3,786
Balance at Dec. 31, 2017	$ 49,941	6,192	34,332	$ 367	$ 5		90,837
Balance (in shares) at Dec. 31, 2017				36,749,072	50,836,629	1
Increase (Decrease) in Members' Equity (Deficit)
Equity-based compensation	6,347						6,347
Exercise of stock options	142						142
Exercise of stock options (in shares)				6,000
Non-controlling interest adjustment for capital contribution of proceeds from the exercise of stock options	(82)		82
Vesting of restricted stock units	29		(30)	$ 1
Vesting of restricted stock units (in shares)				79,000
Disgorgement of short-swing profits by Section 16 officer	557						557
Redemption of LLC common units for Class A common stock	1,847		(116)	$ 2			1,733
Redemption of LLC common units for Class A common stock (in shares)				173,000	(130,000)
Distributions to holders of LLC common units			(71,680)				(71,680)
Dividends		(11,327)					(11,327)
Establishment of liabilities under the Tax Receivable Agreement and related changes to deferred tax assets associated with that liability	(1,414)						(1,414)
Non-controlling interest adjustment	(14,215)		14,215
Net income		31,212	67,879	$ 99,091			99,091
Balance at Jun. 30, 2018	$ 43,152	$ 27,387	$ 47,158	$ 370	$ 5		$ 118,072
Balance (in shares) at Jun. 30, 2018				37,007,619	50,706,629	1