Condensed Consolidated Statement of Stockholders' Equity - USD ($) $ in Thousands	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, 2017	$ 451,199	$ 8,400	$ 459	$ (410,934)	$ 2	$ 0	$ 49,126
Beginning balance, Shares at Dec. 31, 2017					17,696,991	1
Net income	0	4,983	0	4,184	$ 0	$ 0	9,167
Distributions to non-controlling unitholders	0	0	0	(4,212)	0	0	(4,212)
Equity-based compensation expense and related dividend equivalents, value	1,268	(48)	0	0	$ 0	$ 0	1,220
Equity-based compensation expense and related dividend equivalents, shares					46,520	0
Dividends to Class A common stockholders	0	(3,547)	0	0	$ 0	$ 0	(3,547)
Change in accumulated other comprehensive income	0	0	(43)	(39)	0	0	(82)
Payroll taxes related to net settled restricted stock units (value)	(564)	0	0	0	$ 0	$ 0	(564)
Payroll taxes related to net settled restricted stock units (in shares)					(10,209)	0
Ending balance, Value at Mar. 31, 2018	451,903	9,788	416	(411,001)	$ 2	$ 0	51,108
Ending balance, Shares at Mar. 31, 2018					17,733,302	1
Beginning balance, Value at Dec. 31, 2017	451,199	8,400	459	(410,934)	$ 2	$ 0	49,126
Beginning balance, Shares at Dec. 31, 2017					17,696,991	1
Net income							39,299
Dividends to Class A common stockholders							(10,645)
Change in accumulated other comprehensive income							(77)
Ending balance, Value at Sep. 30, 2018	457,026	18,412	419	(403,701)	$ 2	$ 0	72,158
Ending balance, Shares at Sep. 30, 2018					17,746,184	1
Beginning balance, Value at Mar. 31, 2018	451,903	9,788	416	(411,001)	$ 2	$ 0	51,108
Beginning balance, Shares at Mar. 31, 2018					17,733,302	1
Net income	0	7,648	0	6,943	$ 0	$ 0	14,591
Distributions to non-controlling unitholders	0	0	0	(3,606)	0	0	(3,606)
Equity-based compensation expense and related dividend equivalents, value	2,162	(65)	0	0	$ 0	$ 0	2,097
Equity-based compensation expense and related dividend equivalents, shares					18,358	0
Dividends to Class A common stockholders	0	(3,549)	0	0	$ 0	$ 0	(3,549)
Change in accumulated other comprehensive income	0	0	(54)	(31)	0	0	(85)
Payroll taxes related to net settled restricted stock units (value)	(331)	0	0	0	$ 0	$ 0	(331)
Payroll taxes related to net settled restricted stock units (in shares)					(5,476)	0
Other	311	0	0	0	$ 0	$ 0	311
Ending balance, Value at Jun. 30, 2018	454,045	13,822	362	(407,695)	$ 2	$ 0	60,536
Ending balance, Shares at Jun. 30, 2018					17,746,184	1
Net income	0	8,139	0	7,402	$ 0	$ 0	15,541
Distributions to non-controlling unitholders	0	0	0	(3,441)	0	0	(3,441)
Equity-based compensation expense and related dividend equivalents, value	2,776	0	0	0	$ 0	$ 0	2,776
Equity-based compensation expense and related dividend equivalents, shares					0	0
Dividends to Class A common stockholders	0	(3,549)	0	0	$ 0	$ 0	(3,549)
Change in accumulated other comprehensive income	0	0	57	33	0	0	90
Payroll taxes related to net settled restricted stock units (value)	0	0	0	0	$ 0	$ 0	0
Payroll taxes related to net settled restricted stock units (in shares)					0	0
Other	205	0	0	0	$ 0	$ 0	205
Ending balance, Value at Sep. 30, 2018	457,026	18,412	419	(403,701)	$ 2	$ 0	72,158
Ending balance, Shares at Sep. 30, 2018					17,746,184	1
Beginning balance, Value at Dec. 31, 2018	460,101	21,138	328	(402,294)	$ 2	$ 0	79,275
Beginning balance, Shares at Dec. 31, 2018					17,754,416	1
Net income	0	4,409	0	3,848	$ 0	$ 0	8,257
Distributions to non-controlling unitholders	0	0	0	(2,693)	0	0	(2,693)
Equity-based compensation expense and related dividend equivalents, value	3,213	(42)	0	0	$ 0	$ 0	3,171
Equity-based compensation expense and related dividend equivalents, shares					70,797	0
Dividends to Class A common stockholders	0	(3,740)	0	0	$ 0	$ 0	(3,740)
Change in accumulated other comprehensive income	0	0	36	33	0	0	69
Payroll taxes related to net settled restricted stock units (value)	(713)	0	0	0	$ 0	$ 0	(713)
Payroll taxes related to net settled restricted stock units (in shares)					(17,265)	0
Ending balance, Value at Mar. 31, 2019	462,601	21,765	364	(401,106)	$ 2	$ 0	83,626
Ending balance, Shares at Mar. 31, 2019					17,807,948	1
Beginning balance, Value at Dec. 31, 2018	460,101	21,138	328	(402,294)	$ 2	$ 0	79,275
Beginning balance, Shares at Dec. 31, 2018					17,754,416	1
Net income							41,654
Dividends to Class A common stockholders							(11,224)
Change in accumulated other comprehensive income							95
Ending balance, Value at Sep. 30, 2019	462,245	31,992	379	(394,208)	$ 2	$ 0	100,410
Ending balance, Shares at Sep. 30, 2019					17,835,719	1
Beginning balance, Value at Mar. 31, 2019	462,601	21,765	364	(401,106)	$ 2	$ 0	83,626
Beginning balance, Shares at Mar. 31, 2019					17,807,948	1
Net income	0	8,570	0	7,563	$ 0	$ 0	16,133
Distributions to non-controlling unitholders	0	0	0	(4,613)	0	0	(4,613)
Equity-based compensation expense and related dividend equivalents, value	182	(1)	0	0	$ 0	$ 0	181
Equity-based compensation expense and related dividend equivalents, shares					1,740	0
Dividends to Class A common stockholders	0	(3,739)	0	0	$ 0	$ 0	(3,739)
Change in accumulated other comprehensive income	0	0	33	32	0	0	65
Payroll taxes related to net settled restricted stock units (value)	(18)	0	0	0	$ 0	$ 0	(18)
Payroll taxes related to net settled restricted stock units (in shares)					(569)	0
Other	290	0	0	0	$ 0	$ 0	290
Ending balance, Value at Jun. 30, 2019	463,055	26,595	397	(398,124)	$ 2	$ 0	91,925
Ending balance, Shares at Jun. 30, 2019					17,809,119	1
Net income	0	9,173	0	8,091	$ 0	$ 0	17,264
Distributions to non-controlling unitholders	0	0	0	(4,154)	0	0	(4,154)
Equity-based compensation expense and related dividend equivalents, value	(888)	(31)	0	0	$ 0	$ 0	(919)
Equity-based compensation expense and related dividend equivalents, shares					30,278	0
Dividends to Class A common stockholders	0	(3,745)	0	0	$ 0	$ 0	(3,745)
Change in accumulated other comprehensive income	0	0	(18)	(21)	0	0	(39)
Payroll taxes related to net settled restricted stock units (value)	(105)	0	0	0	$ 0	$ 0	(105)
Payroll taxes related to net settled restricted stock units (in shares)					(3,678)	0
Other	183	0	0	0	$ 0	$ 0	183
Ending balance, Value at Sep. 30, 2019	$ 462,245	$ 31,992	$ 379	$ (394,208)	$ 2	$ 0	$ 100,410
Ending balance, Shares at Sep. 30, 2019					17,835,719	1