UNAUDITED CONSOLIDATED STATEMENT OF CHANGES IN EQUITY - USD ($) $ in Thousands	Total	Common Class A	Common Class B	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Non-Controlling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment for the Cumulative Effect of Applying ASU 2016-09	$ 1,377					$ 1,377
Adjusted Balance	82,711	$ 173		$ 5,996	$ (25)	23,726	$ 52,841
Beginning Balance at Dec. 31, 2016	81,334	$ 173		5,996	(25)	22,349	52,841
Beginning Balance (in shares) at Dec. 31, 2016		17,340,090	50,461,598
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amortization of Non-Cash Compensation	3,141			801			2,340
Amortization of Non-Cash Compensation (in shares)		24,934	22,410
Issuance of Shares under Equity Incentive Plan	4,413			1,118			3,295
Issuance of Shares under Equity Incentive Plan (in shares)			620,543
Sale of Shares under Equity Incentive Plan	187			47			140
Sale of Shares under Equity Incentive Plan (in shares)			29,426
Directors' Share Grants	382			96			286
Net Income	45,064					7,954	37,110
Foreign Currency Translation Adjustment	104				26		78
Repurchase and Retirement of Class A Common Stock	(708)	$ (1)		(707)
Repurchase and Retirement Shares/Units (in shares)		(79,717)	(2,897)
Repurchase and Retirement of Class B Units	(39)			(10)			(29)
Class A Cash Dividends Declared and Paid ($0.48 per share)	(5,902)					(5,902)
Contributions from Non-Controlling Interests	4,054						4,054
Distributions to Non-Controlling Interests	(39,841)						(39,841)
Other				(285)			285
Ending Balance at Sep. 30, 2017	93,566	$ 172		7,056	1	25,778	60,559
Ending Balance (in shares) at Sep. 30, 2017		17,285,307	51,131,080
Beginning Balance at Dec. 31, 2017	99,289	$ 180		7,915	(5)	24,214	66,985
Beginning Balance (in shares) at Dec. 31, 2017		18,096,554	50,709,673
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amortization of Non-Cash Compensation	3,721			963			2,758
Amortization of Non-Cash Compensation (in shares)		10,000	26,178
Issuance of Shares under Equity Incentive Plan	4,191			1,096			3,095
Issuance of Shares under Equity Incentive Plan (in shares)			300,931
Sale of Shares under Equity Incentive Plan	5,191			1,342			3,849
Sale of Shares under Equity Incentive Plan (in shares)			897,515
Directors' Share Grants	449			116			333
Net Income	54,470					11,257	43,213
Foreign Currency Translation Adjustment	(90)				23		(113)
Repurchase and Retirement of Class A Common Stock	(7,501)	$ (7)		(7,494)
Repurchase and Retirement Shares/Units (in shares)		(742,255)	(3,870)
Repurchase and Retirement of Class B Units	(41)			(11)			(30)
Class A Cash Dividends Declared and Paid ($0.48 per share)	(8,617)					(8,617)
Tax Impact of Transactions with Non-Controlling Shareholders	(420)			(420)
Contributions from Non-Controlling Interests	61						61
Distributions to Non-Controlling Interests	(53,250)						(53,250)
Other				(1,135)			1,135
Ending Balance at Sep. 30, 2018	$ 97,453	$ 173		$ 2,372	$ 18	$ 26,854	$ 68,036
Ending Balance (in shares) at Sep. 30, 2018		17,364,299	51,930,427