Summary of Significant Accounting Policies (Schedule of Stock by Class) (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Class A [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	44,749	49,601	57,079
Share repurchases	(1,302)	(5,386)	(7,790)
Employee and non-employee director stock transactions*	631	534	312
Ending Balance (in shares)	44,078	44,749	49,601
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*	0	0	0
Ending Balance (in shares)	11,484	11,484	11,484