Condensed Consolidated Statement of Stockholders' Equity - USD ($)	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	$ 451,199,000	$ 8,400,000	$ 459,000	$ (410,934,000)	$ 2,000	$ 0	$ 49,126,000
Beginning balance, Shares at Dec. 31, 2017					17,696,991	1
Net income	0	4,983,000	0	4,184,000	$ 0	$ 0	9,167,000
Distributions to non-controlling unitholders	0	0	0	(4,212,000)	0	0	(4,212,000)
Equity-based compensation expense and related dividend equivalents, value	1,268,000	(48,000)	0	0	$ 0	$ 0	1,220,000
Equity-based compensation expense and related dividend equivalents, shares					46,520	0
Dividends to Class A common stockholders	0	(3,547,000)	0	0	$ 0	$ 0	(3,547,000)
Change in accumulated other comprehensive income	0	0	(43,000)	(39,000)	0	0	(82,000)
Payroll taxes related to net settled restricted stock units (value)	(564,000)	0	0	0	$ 0	$ 0	(564,000)
Payroll taxes related to net settled restricted stock units (in shares)					(10,209)	0
Ending balance, Value at Mar. 31, 2018	451,903,000	9,788,000	416,000	(411,001,000)	$ 2,000	$ 0	51,108,000
Ending balance, Shares at Mar. 31, 2018					17,733,302	1
Beginning balance, Value at Dec. 31, 2017	451,199,000	8,400,000	459,000	(410,934,000)	$ 2,000	$ 0	49,126,000
Beginning balance, Shares at Dec. 31, 2017					17,696,991	1
Net income							23,758,000
Dividends to Class A common stockholders							(7,096,000)
Change in accumulated other comprehensive income							(167,000)
Ending balance, Value at Jun. 30, 2018	454,045,000	13,822,000	362,000	(407,695,000)	$ 2,000	$ 0	60,536,000
Ending balance, Shares at Jun. 30, 2018					17,746,184	1
Beginning balance, Value at Mar. 31, 2018	451,903,000	9,788,000	416,000	(411,001,000)	$ 2,000	$ 0	51,108,000
Beginning balance, Shares at Mar. 31, 2018					17,733,302	1
Net income	0	7,648,000	0	6,943,000	$ 0	$ 0	14,591,000
Distributions to non-controlling unitholders	0	0	0	(3,606,000)	0	0	(3,606,000)
Equity-based compensation expense and related dividend equivalents, value	2,162,000	(65,000)	0	0	$ 0	$ 0	2,097,000
Equity-based compensation expense and related dividend equivalents, shares					18,358	0
Dividends to Class A common stockholders	0	(3,549,000)	0	0	$ 0	$ 0	(3,549,000)
Change in accumulated other comprehensive income	0	0	(54,000)	(31,000)	0	0	(85,000)
Payroll taxes related to net settled restricted stock units (value)	(331,000)	0	0	0	$ 0	$ 0	(331,000)
Payroll taxes related to net settled restricted stock units (in shares)					(5,476)	0
Other	311,000	0	0	0	$ 0	$ 0	311,000
Ending balance, Value at Jun. 30, 2018	454,045,000	13,822,000	362,000	(407,695,000)	$ 2,000	$ 0	60,536,000
Ending balance, Shares at Jun. 30, 2018					17,746,184	1
Beginning balance, Value at Dec. 31, 2018	460,101,000	21,138,000	328,000	(402,294,000)	$ 2,000	$ 0	79,275,000
Beginning balance, Shares at Dec. 31, 2018					17,754,416	1
Net income	0	4,409,000	0	3,848,000	$ 0	$ 0	8,257,000
Distributions to non-controlling unitholders	0	0	0	(2,693,000)	0	0	(2,693,000)
Equity-based compensation expense and related dividend equivalents, value	3,213,000	(42,000)	0	0	$ 0	$ 0	3,171,000
Equity-based compensation expense and related dividend equivalents, shares					70,797	0
Dividends to Class A common stockholders	0	(3,740,000)	0	0	$ 0	$ 0	(3,740,000)
Change in accumulated other comprehensive income	0	0	36,000	33,000	0	0	69,000
Payroll taxes related to net settled restricted stock units (value)	(713,000)	0	0	0	$ 0	$ 0	(713,000)
Payroll taxes related to net settled restricted stock units (in shares)					(17,265)	0
Ending balance, Value at Mar. 31, 2019	462,601,000	21,765,000	364,000	(401,106,000)	$ 2,000	$ 0	83,626,000
Ending balance, Shares at Mar. 31, 2019					17,807,948	1
Beginning balance, Value at Dec. 31, 2018	460,101,000	21,138,000	328,000	(402,294,000)	$ 2,000	$ 0	79,275,000
Beginning balance, Shares at Dec. 31, 2018					17,754,416	1
Net income							24,390,000
Dividends to Class A common stockholders							(7,479,000)
Change in accumulated other comprehensive income							134,000
Ending balance, Value at Jun. 30, 2019	463,055,000	26,595,000	397,000	(398,124,000)	$ 2,000	$ 0	91,925,000
Ending balance, Shares at Jun. 30, 2019					17,809,119	1
Beginning balance, Value at Mar. 31, 2019	462,601,000	21,765,000	364,000	(401,106,000)	$ 2,000	$ 0	83,626,000
Beginning balance, Shares at Mar. 31, 2019					17,807,948	1
Net income	0	8,570,000	0	7,563,000	$ 0	$ 0	16,133,000
Distributions to non-controlling unitholders	0	0	0	(4,613,000)	0	0	(4,613,000)
Equity-based compensation expense and related dividend equivalents, value	182,000	(1,000)	0	0	$ 0	$ 0	181,000
Equity-based compensation expense and related dividend equivalents, shares					1,740	0
Dividends to Class A common stockholders	0	(3,739,000)	0	0	$ 0	$ 0	(3,739,000)
Change in accumulated other comprehensive income	0	0	33,000	32,000	0	0	65,000
Payroll taxes related to net settled restricted stock units (value)	(18,000)	0	0	0	$ 0	$ 0	(18,000)
Payroll taxes related to net settled restricted stock units (in shares)					(569)	0
Other	290,000	0	0	0	$ 0	$ 0	290,000
Ending balance, Value at Jun. 30, 2019	$ 463,055,000	$ 26,595,000	$ 397,000	$ (398,124,000)	$ 2,000	$ 0	$ 91,925,000
Ending balance, Shares at Jun. 30, 2019					17,809,119	1