Stock-Based Compensation and Stock Repurchases (Schedule Of Stock Options Outstanding And Exercisable) (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Equity And Stock-Based Compensation [Line Items]
Weighted Average Exercise Price of Options Outstanding, Granted	$ 15.94	$ 11.87
Class A [Member]
Equity And Stock-Based Compensation [Line Items]
Common Stock Options Outstanding, Beginning balance	535,077	486,565	486,565
Common Stock Options Outstanding, Granted	169,762		169,327
Common Stock Options Outstanding, Exercised	(107,750)		(46,815)
Common Stock Options Outstanding, Forfeited	(2,500)		(74,000)
Common Stock Options Outstanding, Ending balance	594,589		535,077	486,565
Weighted Average Exercise Price of Options Outstanding, Beginning price	$ 9.84	$ 8.68	$ 8.68
Weighted Average Exercise Price of Options Outstanding, Granted	15.94		11.87
Weighted Average Exercise Price of Options Outstanding, Exercised	8.94		9.50
Weighted Average Exercise Price of Options Outstanding, Forfeited	6.23		7.02
Weighted Average Exercise Price of Options Outstanding, Ending price	$ 11.76		$ 9.84	$ 8.68
Weighted Average Remaining Years of Contractual Life	3 years 7 days		2 years 7 months 10 days	2 years 10 months 21 days
Aggregate Intrinsic Value, Beginning balance	$ 3,615,191	$ 2,188,011	$ 2,188,011
Aggregate Intrinsic Value, Exercised	595,873		220,002
Aggregate Intrinsic Value, Ending balance	$ 3,007,071		$ 3,615,191	$ 2,188,011