Shared-Based Compensation and Share Repurchases Plans (Summary Of Vested And Unvested Stock Options) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity And Stock-Based Compensation [Line Items]
Weighted average remaining years of contractual life, Vested	2 years 3 months 18 days	1 year 7 months 2 days	2 years 1 month 21 days
Aggregate intrinsic value, Vested	$ 2,202,772	$ 2,584,500	$ 1,401,321
Weighted average remaining years of contractual life, Nonvested	3 years 7 months 13 days	3 years 10 months 13 days	3 years 8 months 19 days
Aggregate intrinsic value, Nonvested	$ 851,552	$ 1,030,691	$ 786,690
Class A [Member]
Equity And Stock-Based Compensation [Line Items]
Vested	186,832	296,500	256,065
Weighted average exercise price, Vested	$ 9.84	$ 7.88	$ 7.64
Unvested	337,757	238,577	230,500
Weighted average exercise price, Nonvested	$ 13.86	$ 12.28	$ 9.83