Shared-Based Compensation and Share Repurchases Plans (Schedule Of Stock Options Outstanding And Exercisable) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity And Stock-Based Compensation [Line Items]
Weighted Average Exercise Price of Options Outstanding, Granted	$ 15.94	$ 11.87	$ 13.30
Weighted Average Remaining Years of Contractual Life	3 years 1 month 24 days	2 years 7 months 10 days	2 years 10 months 21 days	2 years 4 months 24 days
Aggregate Intrinsic Value, Beginning balance	$ 3,615,191	$ 2,188,011	$ 4,197,000
Aggregate Intrinsic Value, Exercised	702,840	220,002	327,170
Aggregate Intrinsic Value, Ending balance	$ 3,054,325	$ 3,615,191	$ 2,188,011	$ 4,197,000
Class A [Member]
Equity And Stock-Based Compensation [Line Items]
Common Stock Options Outstanding, Beginning balance	535,077	486,565	568,250
Common Stock Options Outstanding, Granted	169,762	169,327	112,000
Common Stock Options Outstanding, Exercised	(177,750)	(46,815)	(185,685)
Common Stock Options Outstanding, Expired	(2,500)	(74,000)	(8,000)
Common Stock Options Outstanding, Ending balance	524,589	535,077	486,565	568,250
Weighted Average Exercise Price of Options Outstanding, Beginning price	$ 9.84	$ 8.68	$ 6.88
Weighted Average Exercise Price of Options Outstanding, Granted	15.94	11.87	13.30
Weighted Average Exercise Price of Options Outstanding, Exercised	7.85	9.50	6.09
Weighted Average Exercise Price of Options Outstanding, Expired	6.23	7.02	6.23
Weighted Average Exercise Price of Options Outstanding, Ending price	$ 12.50	$ 9.84	$ 8.68	$ 6.88
Class B [Member]
Equity And Stock-Based Compensation [Line Items]
Common Stock Options Outstanding, Beginning balance			185,100
Common Stock Options Outstanding, Granted
Common Stock Options Outstanding, Exercised			(185,100)
Common Stock Options Outstanding, Ending balance				185,100
Weighted Average Exercise Price of Options Outstanding, Beginning price			$ 9.90
Weighted Average Exercise Price of Options Outstanding, Exercised			$ 9.90
Weighted Average Exercise Price of Options Outstanding, Ending price				$ 9.90