Measured on recurring basis (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2011	Mar. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	$ 457,901	$ 232,219
Marketable Equity Securities Fair Value Disclosure	4,676	5,264
Corporate Bonds and Debt Fair Value Disclosure	742,114	774,104
Government Agency Bonds Fair Value Disclosure	263,486	342,104
Cash Deposit Accounts Fair Value Disclosure	321,947	403,536
Municipal Bonds Fair Value Disclosure	20,867	20,122
Auction Rate Securities Fair Value Disclosure	10,246	10,246
Contingent consideration	14,700
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	779,848	635,755	651,383	703,924
Short-term investments	881,913	823,254
Long-term investments	159,476	328,586
Total assets measured at fair value	1,821,237	1,787,595
Liabilities, Fair Value Disclosure	14,700
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	457,901	232,219
Marketable Equity Securities Fair Value Disclosure	4,676	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	457,901	232,219
Short-term investments	0	782
Long-term investments	4,676	4,482
Total assets measured at fair value	462,577	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	737,489	769,479
Government Agency Bonds Fair Value Disclosure	263,486	342,104
Cash Deposit Accounts Fair Value Disclosure	321,947	403,536
Municipal Bonds Fair Value Disclosure	20,867	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	321,947	403,536
Short-term investments	881,913	822,472
Long-term investments	139,929	309,233
Total assets measured at fair value	1,343,789	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	4,625	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	10,246	10,246
Contingent consideration	14,700
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	14,871
Total assets measured at fair value	14,871	14,871
Liabilities, Fair Value Disclosure	$ 14,700