Condensed Consolidated Statement of Stockholders' Equity - 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
Balance at Dec. 31, 2017		$ 42,520	$ 7,619	$ 21,252	$ 367	$ 5		$ 71,763
Balance (in shares) at Dec. 31, 2017					36,749,000	50,837,000
Increase (Decrease) in Stockholders' Equity
Equity-based compensation		3,218						3,218
Exercise of stock options		137						137
Exercise of stock options (in shares)					6,000
Non-controlling interest adjustment for capital contribution of proceeds from the exercise of stock options		(77)		77
Vesting of restricted stock units (in shares)					2,000
Redemption of LLC common units for Class A common stock		1,848		(115)	$ 2			1,735
Redemption of LLC common units for Class A common stock (in shares)					173,000	(130,000)
Distributions to holders of LLC common units				(19,938)				(19,938)
Dividends			(5,662)					(5,662)
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		(1,592)		1,592
Net income			1,821	11,727				13,548
Balance at Mar. 31, 2018		44,640	5,088	17,071	$ 369	$ 5		67,173
Balance (in shares) at Mar. 31, 2018					36,930,000	50,707,000
Balance at Dec. 31, 2017		42,520	7,619	21,252	$ 367	$ 5		71,763
Balance (in shares) at Dec. 31, 2017					36,749,000	50,837,000
Increase (Decrease) in Stockholders' Equity
Net income								90,680
Balance at Jun. 30, 2018		45,834	24,206	30,344	$ 370	$ 5		100,759
Balance (in shares) at Jun. 30, 2018					37,007,000	50,707,000
Increase (Decrease) in Stockholders' Equity
Adoption of accounting standard (see Note 1 - Summary of Significant Accounting Policies)			1,310	2,476				3,786
Balance at Mar. 31, 2018		44,640	5,088	17,071	$ 369	$ 5		67,173
Balance (in shares) at Mar. 31, 2018					36,930,000	50,707,000
Increase (Decrease) in Stockholders' Equity
Equity-based compensation		3,129						3,129
Exercise of stock options		5						5
Non-controlling interest adjustment for capital contribution of proceeds from the exercise of stock options		(5)		5
Vesting of restricted stock units		29		(30)	$ 1
Vesting of restricted stock units (in shares)					77,000
Disgorgement of short-swing profits by Section 16 officer		557						557
Distributions to holders of LLC common units				(41,573)				(41,573)
Dividends			(5,664)					(5,664)
Non-controlling interest adjustment		(2,521)		2,521
Net income			24,782	52,350				77,132
Balance at Jun. 30, 2018		45,834	24,206	30,344	$ 370	$ 5		100,759
Balance (in shares) at Jun. 30, 2018					37,007,000	50,707,000
Balance at Dec. 31, 2018		47,531	(3,370)	(11,621)	$ 372	$ 5		32,917
Balance (in shares) at Dec. 31, 2018					37,192,364	50,706,629	1
Increase (Decrease) in Stockholders' Equity
Equity-based compensation		2,716						2,716
Vesting of restricted stock units (in shares)					1,000
Redemption of LLC common units for Class A common stock		12						12
Redemption of LLC common units for Class A common stock (in shares)					6,000
Distributions to holders of LLC common units				(5,534)				(5,534)
Dividends	[1]		(5,699)					(5,699)
Establishment of liabilities under the Tax Receivable Agreement and related changes to deferred tax assets associated with that liability		(8)						(8)
Non-controlling interest adjustment		(1,678)		1,678
Net income			(19,395)	(7,412)				(26,807)
Balance at Mar. 31, 2019		48,573	(24,759)	(16,557)	$ 372	$ 5		7,634
Balance (in shares) at Mar. 31, 2019					37,199,000	50,707,000
Balance at Dec. 31, 2018		47,531	(3,370)	(11,621)	$ 372	$ 5		32,917
Balance (in shares) at Dec. 31, 2018					37,192,364	50,706,629	1
Increase (Decrease) in Stockholders' Equity
Net income								25,816
Balance at Jun. 30, 2019		50,604	(12,453)	(12,916)	$ 373	$ 5		25,613
Balance (in shares) at Jun. 30, 2019					37,272,505	50,706,629	1
Increase (Decrease) in Stockholders' Equity
Adoption of accounting standard (see Note 1 - Summary of Significant Accounting Policies)			3,705	6,332				10,037
Balance at Mar. 31, 2019		48,573	(24,759)	(16,557)	$ 372	$ 5		7,634
Balance (in shares) at Mar. 31, 2019					37,199,000	50,707,000
Increase (Decrease) in Stockholders' Equity
Equity-based compensation		3,863						3,863
Vesting of restricted stock units		143		(144)	$ 1
Vesting of restricted stock units (in shares)					96,000
Repurchases of Class A common stock for withholding taxes on vested RSUs		(273)						(273)
Repurchases of Class A common stock for withholding taxes on vested RSUs (in shares)					(22,000)
Distributions to holders of LLC common units				(32,523)				(32,523)
Dividends	[1]		(5,711)					(5,711)
Non-controlling interest adjustment		(1,702)		1,702
Net income			18,017	34,606				52,623
Balance at Jun. 30, 2019		$ 50,604	$ (12,453)	$ (12,916)	$ 373	$ 5		$ 25,613
Balance (in shares) at Jun. 30, 2019					37,272,505	50,706,629	1

[1] The Company declared dividends per share of Class A common stock of $0.15 for each of the three months ended March 31 and June 30, 2019.