Condensed Consolidated Statements of Stockholders' Equity - USD ($)	Common Stock Common Class A	Common Stock Common Class B	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss), net of tax	Non-controlling interest	Common Class A	Common Class B	Total
Beginning balance, Value at Dec. 31, 2018	$ 2,000	$ 0	$ 460,101,000	$ 21,138,000	$ 328,000	$ (402,294,000)			$ 79,275,000
Beginning balance, Shares at Dec. 31, 2018	17,754,416	1
Net income	$ 0	$ 0	0	4,409,000	0	3,848,000			8,257,000
Distributions to non-controlling unitholders	0	0	0	0	0	(2,693,000)			(2,693,000)
Equity-based compensation expense and related dividend equivalents, value	$ 0	$ 0	3,213,000	(42,000)	0	0			3,171,000
Equity-based compensation expense and related dividend equivalents, shares	70,797	0
Dividends to Class A common stockholders	$ 0	$ 0	0	(3,740,000)	0	0			(3,740,000)
Change in accumulated other comprehensive income	0	0	0	0	36,000	33,000			69,000
Payroll taxes related to net settled restricted stock units, value	$ 0	$ 0	(713,000)	0	0	0			(713,000)
Payroll taxes related to net settled restricted stock units, shares	(17,265)	0
Ending balance, Value at Mar. 31, 2019	$ 2,000	$ 0	462,601,000	21,765,000	364,000	(401,106,000)			83,626,000
Ending balance, Shares at Mar. 31, 2019	17,807,948	1
Beginning balance, Value at Dec. 31, 2018	$ 2,000	$ 0	460,101,000	21,138,000	328,000	(402,294,000)			79,275,000
Beginning balance, Shares at Dec. 31, 2018	17,754,416	1
Net income									24,390,000
Change in accumulated other comprehensive income									134,000
Ending balance, Value at Jun. 30, 2019	$ 2,000	$ 0	463,055,000	26,595,000	397,000	(398,124,000)			91,925,000
Ending balance, Shares at Jun. 30, 2019	17,809,119	1
Beginning balance, Value at Mar. 31, 2019	$ 2,000	$ 0	462,601,000	21,765,000	364,000	(401,106,000)			83,626,000
Beginning balance, Shares at Mar. 31, 2019	17,807,948	1
Net income	$ 0	$ 0	0	8,570,000	0	7,563,000			16,133,000
Distributions to non-controlling unitholders	0	0	0	0	0	(4,613,000)			(4,613,000)
Equity-based compensation expense and related dividend equivalents, value	$ 0	0	182,000	(1,000)	0	0			181,000
Equity-based compensation expense and related dividend equivalents, shares	1,740
Dividends to Class A common stockholders	$ 0	0	0	(3,739,000)	0	0			(3,739,000)
Change in accumulated other comprehensive income	0	0	0	0	33,000	32,000			65,000
Payroll taxes related to net settled restricted stock units, value	$ 0	0	(18,000)	0	0	0			(18,000)
Payroll taxes related to net settled restricted stock units, shares	(569)
Other	$ 0	0	290,000	0	0	0			290,000
Ending balance, Value at Jun. 30, 2019	$ 2,000	$ 0	463,055,000	26,595,000	397,000	(398,124,000)			91,925,000
Ending balance, Shares at Jun. 30, 2019	17,809,119	1
Beginning balance, Value at Dec. 31, 2019	$ 2,000	$ 0	466,945,000	30,525,000	414,000	(399,510,000)			98,376,000
Beginning balance, Shares at Dec. 31, 2019	17,838,233	1					17,838,233	1
Net income	$ 0	$ 0	0	2,631,000	0	2,659,000			5,290,000
Distributions to non-controlling unitholders	0	0	0	0	0	(2,777,000)			(2,777,000)
Equity-based compensation expense and related dividend equivalents, value	$ 0	$ 0	5,962,000	(289,000)	0	0			5,673,000
Equity-based compensation expense and related dividend equivalents, shares	368,375	0
Dividends to Class A common stockholders	$ 0	$ 0	0	(3,986,000)	0	0			(3,986,000)
Change in accumulated other comprehensive income	0	0	0	0	(36,000)	(194,000)			(230,000)
Payroll taxes related to net settled restricted stock units, value	$ 0	$ 0	(2,268,000)	0	0	0			(2,268,000)
Payroll taxes related to net settled restricted stock units, shares	(82,645)	0
Ending balance, Value at Mar. 31, 2020	$ 2,000	$ 0	470,639,000	28,881,000	378,000	(399,822,000)			100,078,000
Ending balance, Shares at Mar. 31, 2020	18,123,963	1
Beginning balance, Value at Dec. 31, 2019	$ 2,000	$ 0	466,945,000	30,525,000	414,000	(399,510,000)			98,376,000
Beginning balance, Shares at Dec. 31, 2019	17,838,233	1					17,838,233	1
Net income									11,214,000
Change in accumulated other comprehensive income									(113,000)
Ending balance, Value at Jun. 30, 2020	$ 2,000	$ 0	473,451,000	28,385,000	440,000	(400,121,000)			102,157,000
Ending balance, Shares at Jun. 30, 2020	18,123,963	1					18,123,963	1
Beginning balance, Value at Mar. 31, 2020	$ 2,000	$ 0	470,639,000	28,881,000	378,000	(399,822,000)			100,078,000
Beginning balance, Shares at Mar. 31, 2020	18,123,963	1
Net income	$ 0	$ 0	0	3,489,000	0	2,435,000			5,924,000
Distributions to non-controlling unitholders	0	0	0	0	0	(2,789,000)			(2,789,000)
Equity-based compensation expense and related dividend equivalents, value	0	0	2,812,000	0	0	0			2,812,000
Dividends to Class A common stockholders	0	0	0	(3,987,000)	0	0			(3,987,000)
Change in accumulated other comprehensive income	0	0	0	0	62,000	55,000			117,000
Other	0	0	0	2,000	0	0			2,000
Ending balance, Value at Jun. 30, 2020	$ 2,000	$ 0	$ 473,451,000	$ 28,385,000	$ 440,000	$ (400,121,000)			$ 102,157,000
Ending balance, Shares at Jun. 30, 2020	18,123,963	1					18,123,963	1