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Marketable Securities (Schedule of Available-for-sale Securities) (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 82,350 $ 118,783
Gross Unrealized Gains 31 47
Gross Unrealized Losses (22) (39)
Estimated Fair Value (Net Carrying Amount) 82,359 118,791
Corporate Debt Securities [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 32,554 48,011
Gross Unrealized Gains 21 33
Gross Unrealized Losses (12) (19)
Estimated Fair Value (Net Carrying Amount) 32,563 48,025
U.S. Treasury Securities [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 16,762 30,264
Gross Unrealized Gains 2 1
Gross Unrealized Losses (2) (4)
Estimated Fair Value (Net Carrying Amount) 16,762 30,261
Agency Discount Notes [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 17,028 16,789
Gross Unrealized Gains 4 8
Gross Unrealized Losses   (1)
Estimated Fair Value (Net Carrying Amount) 17,032 16,796
Commercial Paper [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 16,006 23,719
Gross Unrealized Gains 4 5
Gross Unrealized Losses (8) (15)
Estimated Fair Value (Net Carrying Amount) $ 16,002 $ 23,709