Earnings per Share - Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Diluted Net Income Allocated to:
Total Diluted Net Income Attributable to Shareholders		$ 15,786	$ 11,862	$ 44,033	$ 31,077
Total Basic Weighted-Average Shares Outstanding (in shares)	[1]	17,489,380	17,285,307	17,752,788	17,319,948
Dilutive Effect of B Units (in shares)		51,930,114	51,123,950	51,537,499	51,108,273
Dilutive Effect of Options (in shares)		691,159	579,881	1,071,344	474,785
Dilutive Effect of Phantom Class B Units & Phantom Shares of Class A Common Stock (in shares)		1,461,725	1,636,784	1,463,747	1,808,570
Dilutive Effect of Restricted Shares of Class A Common Stock (in shares)		56,828	72,405	60,433	69,430
Diluted Weighted Average Common Shares Outstanding (in shares)		71,629,206	70,698,327	71,885,811	70,781,006
Participating Class B Units (in shares)		48,600	64,886	48,600	64,886
Diluted Weighted-Average Shares Outstanding (in shares)	[1]	71,677,806	70,763,213	71,934,411	70,845,892
Diluted Earnings per Share (in dollars per share)		$ 0.22	$ 0.17	$ 0.61	$ 0.44
Options to Purchase Class B Units
Diluted Net Income Allocated to:
Antidilutive securities excluded from diluted EPS (in shares)		400,000	700,000	400,000	700,000
Options to Purchase Shares of Class A Common Stock
Diluted Net Income Allocated to:
Antidilutive securities excluded from diluted EPS (in shares)		100,000	100,000	100,000	100,000
Contingent Options to Purchase Shares of Class A Common Stock
Diluted Net Income Allocated to:
Antidilutive securities excluded from diluted EPS (in shares)		2,000,000	2,000,000	2,000,000	2,000,000
Common Class A
Diluted Net Income Allocated to:
Total Diluted Net Income Attributable to Shareholders		$ 15,775	$ 11,851	$ 44,002	$ 31,047
Total Basic Weighted-Average Shares Outstanding (in shares)		17,489,380	17,285,307	17,752,788	17,316,392
Participating Shares of Restricted Class A Common Stock
Diluted Net Income Allocated to:
Total Diluted Net Income Attributable to Shareholders					$ 2
Total Basic Weighted-Average Shares Outstanding (in shares)		0	0	0	3,556
Participating Class B Units
Diluted Net Income Allocated to:
Total Diluted Net Income Attributable to Shareholders		$ 11	$ 11	$ 31	$ 28

[1] The Company issues restricted shares of Class A common stock and restricted Class B units that have non-forfeitable dividend rights. Under the "two-class method," these shares and units are considered participating securities and are required to be included in the computation of basic and diluted earnings per share.