Marketable Investments (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	$ 176,774	$ 153,342
Level 1 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	815	5,786
Level 2 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	166,989	137,991
Level 3 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	8,970	9,565
Money market funds [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	815	5,786
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	815	5,786
Money market funds [Member] | Level 2 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
Money market funds [Member] | Level 3 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
State and municipal obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	18,872	9,482
State and municipal obligations [Member] | Level 1 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
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	18,872	9,482
State and municipal obligations [Member] | Level 3 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
Federal agency and corporate obligations [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	148,117	128,509
Federal agency and corporate obligations [Member] | Level 1 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
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	148,117	128,509
Federal agency and corporate obligations [Member] | Level 3 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
ARS [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	8,970	9,565
ARS [Member] | Level 1 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
ARS [Member] | Level 2 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis
ARS [Member] | Level 3 [Member]
Summary of Company's fair value hierarchy for its financial assets
Fair value of assets on recurring basis	$ 8,970	$ 9,565