Summary of Significant Accounting Policies (Schedule of Stock by Class) (Details) - shares shares in Thousands			12 Months Ended
		Oct. 21, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Common Class A [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)			44,078	44,749	49,601
Share repurchases		(10,800)	(14,785)	(1,302)	(5,386)
Employee and non-employee director stock transactions*	[1]		682	631	534
Ending Balance (in shares)			29,975	44,078	44,749
Common Class B [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)			11,484	11,484	11,484
Share repurchases			0	0	0
Employee and non-employee director stock transactions*	[1]		0	0	0
Ending Balance (in shares)			11,484	11,484	11,484

[1] *Reflects common stock activity in connection with restricted stock units and stock options granted to employees, as well as in connection with the fulfillment of employees' statutory tax withholding obligations for applicable income and other employment taxes and forfeited employee restricted stock units.