Measured on recurring basis (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2010	Mar. 31, 2009
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	$ 232,219	$ 464,669
Marketable Equity Securities Fair Value Disclosure	5,264	26,861
Corporate Bonds and Debt Fair Value Disclosure	774,104	523,026
Government Agency Bonds Fair Value Disclosure	342,104	430,591
Cash Deposit Accounts Fair Value Disclosure	403,536	239,255
Municipal Bonds Fair Value Disclosure	20,122	11,457
Auction Rate Securities Fair Value Disclosure	10,246	12,475
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	635,755	703,924	492,130	446,329
Short-term investments	823,254	539,572
Long-term investments	328,586	464,838
Total assets measured at fair value	1,787,595	1,708,334
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	232,219	464,669
Marketable Equity Securities Fair Value Disclosure	5,264	26,861
Corporate Bonds and Debt Fair Value Disclosure	0	0
Government Agency Bonds Fair Value Disclosure	0	0
Cash Deposit Accounts Fair Value Disclosure	0	0
Municipal Bonds Fair Value Disclosure	0	0
Auction Rate Securities Fair Value Disclosure	0	0
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	232,219	464,669
Short-term investments	782	16,553
Long-term investments	4,482	10,308
Total assets measured at fair value	237,483	491,530
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	0	0
Marketable Equity Securities Fair Value Disclosure	0	0
Corporate Bonds and Debt Fair Value Disclosure	769,479	519,526
Government Agency Bonds Fair Value Disclosure	342,104	430,591
Cash Deposit Accounts Fair Value Disclosure	403,536	239,255
Municipal Bonds Fair Value Disclosure	20,122	11,457
Auction Rate Securities Fair Value Disclosure	0	0
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	403,536	239,255
Short-term investments	822,472	523,019
Long-term investments	309,233	438,555
Total assets measured at fair value	1,535,241	1,200,829
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	0	0
Marketable Equity Securities Fair Value Disclosure	0	0
Corporate Bonds and Debt Fair Value Disclosure	4,625	3,500
Government Agency Bonds Fair Value Disclosure	0	0
Cash Deposit Accounts Fair Value Disclosure	0	0
Municipal Bonds Fair Value Disclosure	0	0
Auction Rate Securities Fair Value Disclosure	10,246	12,475
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	0	0
Short-term investments	0	0
Long-term investments	14,871	15,975
Total assets measured at fair value	$ 14,871	$ 15,975