Measured on recurring basis (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010	Mar. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	$ 237,653	$ 464,669
Marketable Equity Securities Fair Value Disclosure	6,233	26,861
Corporate Bonds and Debt Fair Value Disclosure	723,261	523,026
Government Agency Bonds Fair Value Disclosure	405,700	430,591
Cash Deposit Accounts Fair Value Disclosure	368,072	239,255
Municipal Bonds Fair Value Disclosure	17,749	11,457
Auction Rate Securities Fair Value Disclosure	11,691	12,475
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	605,725	703,924	500,616	492,130
Short-term investments	696,548	539,572
Long-term investments	468,086	464,838
Total assets measured at fair value	1,770,359	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	237,653	464,669
Marketable Equity Securities Fair Value Disclosure	6,233	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	237,653	464,669
Short-term investments	1,550	16,553
Long-term investments	4,683	10,308
Total assets measured at fair value	243,886	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	718,636	519,526
Government Agency Bonds Fair Value Disclosure	405,700	430,591
Cash Deposit Accounts Fair Value Disclosure	368,072	239,255
Municipal Bonds Fair Value Disclosure	17,749	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	368,072	239,255
Short-term investments	694,998	523,019
Long-term investments	447,087	438,555
Total assets measured at fair value	1,510,157	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	11,691	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	16,316	15,975
Total assets measured at fair value	$ 16,316	$ 15,975