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Marketable Securities (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Summary of available-for-sale securities    
Available-for-sale securities, Cost $ 68,483 $ 68,164
Available-for-sale securities, Estimated Fair Value 69,035 68,566
Level 1 [Member] | Certificates of deposit [Member]
    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 1,902 3,646
Unrecognized losses for less than 12 months, Estimated Fair value 1,893 3,605
Unrecognized losses for more than 12 months, Cost 103  
Unrecognized losses for more than 12 months, Estimated Fair value 103  
Unrecognized gains, Cost 4,324 3,525
Unrecognized gains, Estimated Fair value 4,360 3,551
Available-for-sale securities, Cost 6,329 7,171
Available-for-sale securities, Estimated Fair Value 6,356 7,156
Level 2 [Member] | U.S. Treasury and agency notes [Member]
    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 6,293 4,352
Unrecognized losses for less than 12 months, Estimated Fair value 6,236 4,338
Unrecognized losses for more than 12 months, Cost   1,002
Unrecognized losses for more than 12 months, Estimated Fair value   1,000
Unrecognized gains, Cost 14,723 16,479
Unrecognized gains, Estimated Fair value 14,844 16,660
Available-for-sale securities, Cost 21,016 21,833
Available-for-sale securities, Estimated Fair Value 21,080 21,998
Level 2 [Member] | Corporate notes [Member]
    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 4,739 9,230
Unrecognized losses for less than 12 months, Estimated Fair value 4,707 9,098
Unrecognized losses for more than 12 months, Cost 821 1,539
Unrecognized losses for more than 12 months, Estimated Fair value 818 1,494
Unrecognized gains, Cost 26,052 19,369
Unrecognized gains, Estimated Fair value 26,460 19,738
Available-for-sale securities, Cost 31,612 30,138
Available-for-sale securities, Estimated Fair Value 31,985 30,330
Level 2 [Member] | Municipal notes [Member]
    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 1,058 1,177
Unrecognized losses for less than 12 months, Estimated Fair value 1,048 1,165
Unrecognized losses for more than 12 months, Cost 952 881
Unrecognized losses for more than 12 months, Estimated Fair value 944 873
Unrecognized gains, Cost 7,516 6,964
Unrecognized gains, Estimated Fair value 7,622 7,044
Available-for-sale securities, Cost 9,526 9,022
Available-for-sale securities, Estimated Fair Value $ 9,614 $ 9,082