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INVESTMENTS (Details) (USD $)
In Thousands, unless otherwise specified
6 Months Ended 12 Months Ended
Sep. 30, 2013
Mar. 31, 2013
Held-to-maturity securities:    
Estimated Fair Value $ 1,200  
Total investments    
Amortized Cost 213,780 207,361
Gross Unrealized Gains 8,190 64
Gross Unrealized Losses, Less than One Year (78) (45)
Gross Unrealized Losses, Greater than One Year (156) (162)
Estimated Fair Value 221,736 207,218
Short-term investments
   
Available-for-sale securities:    
Amortized Cost 164,598 123,128
Gross Unrealized Gains 139 64
Gross Unrealized Losses, Less than One Year (17) (2)
Estimated Fair Value 164,720 123,190
Total investments    
Amortized Cost 165,799 124,329
Gross Unrealized Gains 139 64
Gross Unrealized Losses, Less than One Year (17) (2)
Estimated Fair Value 165,921 124,391
Short-term investments | Money market funds
   
Total investments    
Amortized Cost 1,201 1,201
Estimated Fair Value 1,201 1,201
Short-term investments | U.S. government and agency debt securities
   
Available-for-sale securities:    
Amortized Cost 132,502 102,093
Gross Unrealized Gains 84 29
Gross Unrealized Losses, Less than One Year (15) (1)
Estimated Fair Value 132,571 102,121
Short-term investments | Corporate debt securities
   
Available-for-sale securities:    
Amortized Cost 18,641 10,946
Gross Unrealized Gains 44 27
Estimated Fair Value 18,685 10,973
Short-term investments | International government agency debt securities
   
Available-for-sale securities:    
Amortized Cost 13,455 10,089
Gross Unrealized Gains 11 8
Gross Unrealized Losses, Less than One Year (2) (1)
Estimated Fair Value 13,464 10,096
Long-term investments
   
Available-for-sale securities:    
Amortized Cost 46,781 81,832
Gross Unrealized Gains 8,051  
Gross Unrealized Losses, Less than One Year (61) (43)
Gross Unrealized Losses, Greater than One Year (156) (162)
Estimated Fair Value 54,615 81,627
Held-to-maturity securities:    
Amortized Cost 47,981 83,032
Gross Unrealized Gains 8,051  
Gross Unrealized Losses, Less than One Year (61) (43)
Gross Unrealized Losses, Greater than One Year (156) (162)
Estimated Fair Value 55,815 82,827
Long-term investments | Certificates of deposit
   
Held-to-maturity securities:    
Amortized Cost 1,200 1,200
Estimated Fair Value 1,200 1,200
Long-term investments | U.S. government and agency debt securities
   
Available-for-sale securities:    
Amortized Cost 14,799 60,047
Gross Unrealized Losses, Less than One Year (26) (17)
Gross Unrealized Losses, Greater than One Year (11)  
Estimated Fair Value 14,762 60,030
Long-term investments | Corporate debt securities
   
Available-for-sale securities:    
Amortized Cost 17,207 18,725
Gross Unrealized Losses, Less than One Year (31) (26)
Gross Unrealized Losses, Greater than One Year (145) (162)
Estimated Fair Value 17,031 18,537
Long-term investments | Equity securities
   
Available-for-sale securities:    
Amortized Cost 8,732  
Gross Unrealized Gains 8,051  
Estimated Fair Value 16,783  
Long-term investments | International government agency debt securities
   
Available-for-sale securities:    
Amortized Cost 6,043 3,060
Gross Unrealized Losses, Less than One Year (4)  
Estimated Fair Value $ 6,039 $ 3,060