Marketable Investments (Details 3) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	$ 169,888	$ 153,342
Money market funds [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	2,246	5,786
State and municipal obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	19,076	9,482
Federal agency and corporate obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	138,777	128,509
ARS [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	9,789	9,565
Level 1 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	2,246	5,786
Level 1 [Member] | Money market funds [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	2,246	5,786
Level 1 [Member] | State and municipal obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
Level 1 [Member] | Federal agency and corporate obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
Level 1 [Member] | ARS [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
Level 2 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	157,853	137,991
Level 2 [Member] | Money market funds [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
Level 2 [Member] | State and municipal obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	19,076	9,482
Level 2 [Member] | Federal agency and corporate obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	138,777	128,509
Level 2 [Member] | ARS [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
Level 3 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	9,789	9,565
Level 3 [Member] | Money market funds [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
Level 3 [Member] | State and municipal obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
Level 3 [Member] | Federal agency and corporate obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
Level 3 [Member] | ARS [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	$ 9,789	$ 9,565