Stock-Based Compensation and Stock Repurchases (Summary Of Vested Stock Options) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity And Stock-Based Compensation [Line Items]
Weighted average remaining years of contractual life, Vested	1 year 7 months 2 days	2 years 1 month 21 days	3 years 7 months 17 days
Aggregate intrinsic value, Vested	$ 2,584,500	$ 1,401,321	$ 2,476,230
Weighted average remaining years of contractual life, Nonvested	3 years 10 months 13 days	3 years 8 months 19 days	4 years 3 months
Aggregate intrinsic value, Nonvested	$ 1,030,691	$ 786,690	$ 1,720,770
Class A [Member]
Equity And Stock-Based Compensation [Line Items]
Number of options, Vested	296,500	256,065	348,000
Weighted average exercise price, Vested	$ 7.88	$ 7.64	$ 6.82
Number of options, Nonvested	238,577	230,500	220,250
Weighted average exercise price, Nonvested	$ 12.28	$ 9.83	$ 6.98
Class B [Member]
Equity And Stock-Based Compensation [Line Items]
Number of options, Vested			185,100
Weighted average exercise price, Vested			$ 9.90