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CASH, CASH EQUIVALENTS AND INVESTMENTS (Details) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Mar. 31, 2012
Sep. 30, 2011
Mar. 31, 2011
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Cash and Cash Equivalents, Fair Value $ 207,929 $ 209,335 $ 185,377 $ 284,375
Cash, Cash Equivalents Before Unrealized Gains And Losses, and Amortized Cost Basis of Available-For-Sale Securities 399,044 389,777    
Cash Equivalents and Available-For-Sale Securities, Gross Unrealized Gains 166 141    
Cash Equivalents and Available-For-Sale Securities, Gross Unrealized Losses (6) (59)    
Cash and Fair Value of Cash Equivalents and Available-For-Sale Securities 399,204 389,859    
Available for Sale Securities, Debt Maturities [Abstract]        
Due in 1 year or less (amortized cost) 237,541 187,118    
Due in 1 to 3 years (amortized cost) 38,713 55,321    
Due in 1 year or less (fair value) 237,639 187,174    
Due in 1 to 3 years (fair value) 38,775 55,347    
Total amortized cost 276,254 242,439    
Total fair value 276,414 242,521    
U.S. Treasury Bills and Government Agency Securities [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Available-for-sale securities, amortized cost basis 24,154 29,814    
Available-for-sale securities, gross unrealized gains 26 24    
Available-for-sale securities, gross unrealized losses 0 (1)    
Available-for-sale securities, fair value 24,180 29,837    
Corporate Bonds [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Available-for-sale securities, amortized cost basis 13,556 25,507    
Available-for-sale securities, gross unrealized gains 33 29    
Available-for-sale securities, gross unrealized losses 0 (26)    
Available-for-sale securities, fair value 13,589 25,510    
Certificates of Deposit [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Available-for-sale securities, amortized cost basis 1,003 0    
Available-for-sale securities, gross unrealized gains 3 0    
Available-for-sale securities, gross unrealized losses 0 0    
Available-for-sale securities, fair value 1,006 0    
U.S. Treasury Bills and Government Agency Securities [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Available-for-sale securities, amortized cost basis 75,270 61,898    
Available-for-sale securities, gross unrealized gains 29 22    
Available-for-sale securities, gross unrealized losses (4) (24)    
Available-for-sale securities, fair value 75,295 61,896    
Commercial Paper [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Available-for-sale securities, amortized cost basis 25,715 20,041    
Available-for-sale securities, gross unrealized gains 8 1    
Available-for-sale securities, gross unrealized losses (2) (3)    
Available-for-sale securities, fair value 25,721 20,039    
Corporate Bonds [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Available-for-sale securities, amortized cost basis 46,535 38,300    
Available-for-sale securities, gross unrealized gains 66 60    
Available-for-sale securities, gross unrealized losses 0 (4)    
Available-for-sale securities, fair value 46,601 38,356    
Certificates of Deposit [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Available-for-sale securities, amortized cost basis 4,882 4,883    
Available-for-sale securities, gross unrealized gains 1 3    
Available-for-sale securities, gross unrealized losses 0 0    
Available-for-sale securities, fair value 4,883 4,886    
Cash [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Cash 122,790 147,338    
Cash Equivalents [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Cash Equivalents Before Unrealized Gains And Losses 85,139 61,996    
Cash Equivalents, Gross Unrealized Gains 0 2    
Cash Equivalents, Gross Unrealized Losses 0 (1)    
Cash equivalents, fair value 85,139 61,997    
Cash and cash equivalents [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Cash and Cash Equivalents Before Unrealized Gains And Losses 207,929 209,334    
Cash Equivalents, Gross Unrealized Gains 0 2    
Cash Equivalents, Gross Unrealized Losses 0 (1)    
Cash and Cash Equivalents, Fair Value 207,929 209,335    
Short-term Investments [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Available-for-sale securities, amortized cost basis 152,402 125,122    
Available-for-sale securities, gross unrealized gains 104 86    
Available-for-sale securities, gross unrealized losses (6) (31)    
Available-for-sale securities, fair value 152,500 125,177    
Long-term investments [Member]
       
Schedule of Cash, Cash Equivalents and Investments [Line Items]        
Available-for-sale securities, amortized cost basis 38,713 55,321    
Available-for-sale securities, gross unrealized gains 62 53    
Available-for-sale securities, gross unrealized losses 0 (27)    
Available-for-sale securities, fair value $ 38,775 $ 55,347