Summary of Significant Accounting Policies (Schedule of Stock by Class) (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Class A [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	57,079,039	60,910,000	60,553,000
Employee and non-employee director stock transactions	312,000	289,000	357,000
Share repurchases	(7,800,000)	(4,120,000)
Ending Balance (in shares)	49,601,184	57,079,039	60,910,000
Common Class B [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	11,484,408	11,484,000	11,484,000
Employee and non-employee director stock transactions	0	0	0
Share repurchases	0	0
Ending Balance (in shares)	11,484,408	11,484,408	11,484,000