Stock-Based Compensation and Stock Repurchases (Schedule Of Stock Options Outstanding And Exercisable) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Equity And Stock-Based Compensation [Line Items]
Weighted Average Exercise Price of Options Outstanding, Granted		$ 15.97
Class A [Member]
Equity And Stock-Based Compensation [Line Items]
Number of Stock Options, Beginning balance	524,589	535,077	535,077
Number of Stock Options, Granted			169,762
Number of Stock Options, Exercised	(30,000)		(177,750)
Number of Stock Options, Forfeited			(2,500)
Common Stock Options Outstanding, Ending balance	494,589		524,589	535,077
Weighted Average Exercise Price of Options Outstanding, Beginning price	$ 12.50	$ 9.84	$ 9.84
Weighted Average Exercise Price of Options Outstanding, Granted			15.94
Weighted Average Exercise Price of Options Outstanding, Exercised	5.74		7.85
Weighted Average Exercise Price of Options Outstanding, Forfeited			6.23
Weighted Average Exercise Price of Options Outstanding, Ending price	$ 12.17		$ 12.50	$ 9.84
Weighted Average Remaining Years of Contractual Life	3 years 26 days		3 years 1 month 24 days	2 years 7 months 10 days
Aggregate Intrinsic Value, Beginning balance	$ 3,054,325	$ 3,615,191	$ 3,615,191
Aggregate Intrinsic Value, Exercised	321,749		702,840
Aggregate Intrinsic Value, Ending balance	$ 2,832,550		$ 3,054,325	$ 3,615,191