Shared-Based Compensation And Share Repurchase Plans (Summary Of Vested And Unvested Stock Options) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity And Stock-Based Compensation [Line Items]
Weighted average remaining years of contractual life, Vested	2 years 3 months 11 days	2 years 3 months 18 days	1 year 7 months 2 days
Aggregate intrinsic value, Vested	$ 1,306,643	$ 2,202,772	$ 2,584,500
Weighted average remaining years of contractual life, Nonvested	3 years 3 months 7 days	3 years 7 months 13 days	3 years 10 months 13 days
Aggregate intrinsic value, Nonvested	$ 223,885	$ 851,552	$ 1,030,691
Class A [Member]
Equity And Stock-Based Compensation [Line Items]
Vested	231,124	186,832	296,500
Weighted average exercise price, Vested	$ 12.38	$ 9.84	$ 7.88
Unvested	355,345	337,757	238,577
Weighted average exercise price, Nonvested	$ 15.07	$ 13.86	$ 12.28