Measured on Recurring Basis (Details) (USD $)	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Mutual Funds	279,833,000	$ 192,159,000
Marketable Equity Securities Fair Value Disclosure	13,166,000
Corporate Bonds and Debt	926,929,000	943,966,000
Time Deposits Fair Value Disclosure	506,000
Government Agency Bonds	742,256,000	681,394,000
Deposit Accounts	327,982,000	274,444,000
Municipal Bonds	41,698,000	41,709,000
Auction Rate Securities	9,825,000	9,825,000
Derivative Assets Fair Value Disclosure	8,928,000
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	607,815,000	466,603,000	528,334,000	635,755,000
Short-term investments	1,351,054,000	878,182,000
Long-term investments	383,326,000	798,712,000
Other Assets, Noncurrent	75,510,000	45,956,000
Total assets measured at fair value	2,351,123,000	2,143,497,000
Insurance Sector Auction Rate Securities
Available-for-sale Securities, Other Disclosure Items [Abstract]
Value of auction rate securities for which recent auctions were unsuccessful	9,800,000
Par Value of auction rate securities for which recent auctions were unsuccessful	22,400,000
Insurance Sector Auction Rate Securities | Minimum
Available-for-sale Securities, Other Disclosure Items [Abstract]
Discount rate	2.00%
Liquidity risk premium (percentage)	9.10%
Anticipated liquidity horizon	7 years
Insurance Sector Auction Rate Securities | Maximum
Available-for-sale Securities, Other Disclosure Items [Abstract]
Discount rate	2.50%
Liquidity risk premium (percentage)	29.50%
Anticipated liquidity horizon	10 years
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Mutual Funds	279,833,000	192,159,000
Marketable Equity Securities Fair Value Disclosure	13,166,000
Corporate Bonds and Debt	0	0
Time Deposits Fair Value Disclosure	0
Government Agency Bonds	0	0
Deposit Accounts	0	0
Municipal Bonds	0	0
Auction Rate Securities	0	0
Derivative Assets Fair Value Disclosure	0
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	279,833,000	192,159,000
Short-term investments	13,166,000	0
Long-term investments	0	0
Other Assets, Noncurrent	0
Total assets measured at fair value	292,999,000	192,159,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Mutual Funds	0	0
Marketable Equity Securities Fair Value Disclosure	0
Corporate Bonds and Debt	920,739,000	937,776,000
Time Deposits Fair Value Disclosure	506,000
Government Agency Bonds	742,256,000	681,394,000
Deposit Accounts	327,982,000	274,444,000
Municipal Bonds	41,698,000	41,709,000
Auction Rate Securities	0	0
Derivative Assets Fair Value Disclosure	8,928,000
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	327,982,000	274,444,000
Short-term investments	1,337,888,000	878,182,000
Long-term investments	367,311,000	782,697,000
Other Assets, Noncurrent	8,928,000
Total assets measured at fair value	2,042,109,000	1,935,323,000
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Mutual Funds	0	0
Marketable Equity Securities Fair Value Disclosure	0
Corporate Bonds and Debt	6,190,000	6,190,000
Time Deposits Fair Value Disclosure	0
Government Agency Bonds	0	0
Deposit Accounts	0	0
Municipal Bonds	0	0
Auction Rate Securities	9,825,000	9,825,000
Derivative Assets Fair Value Disclosure	0
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	16,015,000	16,015,000
Other Assets, Noncurrent	0
Total assets measured at fair value	16,015,000	$ 16,015,000