Marketable Investments (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	$ 194,848	$ 176,774
Level 1 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	15,314	815
Level 2 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	170,608	166,989
Level 3 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	8,926	8,970
Money market funds [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	15,314	815
Money market funds [Member] | Level 1 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	15,314	815
State and municipal obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	18,705	18,872
State and municipal obligations [Member] | Level 2 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	18,705	18,872
Federal agency and corporate obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	151,903	148,117
Federal agency and corporate obligations [Member] | Level 2 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	151,903	148,117
ARS [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	8,926	8,970
ARS [Member] | Level 3 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	$ 8,926	$ 8,970