Summary of Significant Accounting Policies (Schedule of Stock by Class) (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Class A [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	49,601,184	57,079,000	60,910,000
Share repurchases	(5,400,000)	(7,790,000)	(4,120,000)
Employee and non-employee director stock transactions	534,000	312,000	289,000
Ending Balance (in shares)	44,748,605	49,601,184	57,079,000
Common Class B [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	11,484,408	11,484,000	11,484,000
Share repurchases	0	0	0
Employee and non-employee director stock transactions	0	0	0
Ending Balance (in shares)	11,484,408	11,484,408	11,484,000