Measured on recurring basis (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Mar. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	$ 137,836	$ 232,219
Marketable Equity Securities Fair Value Disclosure	4,435	5,264
Corporate Bonds and Debt Fair Value Disclosure	731,367	774,104
Government Agency Bonds Fair Value Disclosure	480,505	342,104
Cash Deposit Accounts Fair Value Disclosure	298,899	403,536
Municipal Bonds Fair Value Disclosure	20,068	20,122
Auction Rate Securities Fair Value Disclosure	34,302	10,246
Contingent consideration	16,632
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	436,735	635,755	675,129	703,924
Short-term investments	1,116,675	823,254
Long-term investments	154,002	328,586
Total assets measured at fair value	1,707,412	1,787,595
Liabilities, Fair Value Disclosure	16,632
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	137,836	232,219
Marketable Equity Securities Fair Value Disclosure	4,435	5,264
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
Contingent consideration	0
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	137,836	232,219
Short-term investments	0	782
Long-term investments	4,435	4,482
Total assets measured at fair value	142,271	237,483
Liabilities, Fair Value Disclosure	0
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	725,686	769,479
Government Agency Bonds Fair Value Disclosure	480,505	342,104
Cash Deposit Accounts Fair Value Disclosure	298,899	403,536
Municipal Bonds Fair Value Disclosure	20,068	20,122
Auction Rate Securities Fair Value Disclosure	0	0
Contingent consideration	0
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	298,899	403,536
Short-term investments	1,116,675	822,472
Long-term investments	109,584	309,233
Total assets measured at fair value	1,525,158	1,535,241
Liabilities, Fair Value Disclosure	0
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	5,681	4,625
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	34,302	10,246
Contingent consideration	16,632
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	39,983	14,871
Total assets measured at fair value	39,983	14,871
Liabilities, Fair Value Disclosure	$ 16,632