Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	$ 3,076,976	$ 3,063,038
Derivative financial instruments, net	5,611	3,070
Fair Value Liabilities, Convertible debentures - embedded derivative	1,545	931
Total liabilities measured at fair value	7,156	4,001
Net assets measured at fair value	3,069,820	3,059,037
Fair Value, Inputs, Level 1 [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	657,230	647,355
Derivative financial instruments, net	0	0
Fair Value Liabilities, Convertible debentures - embedded derivative	0	0
Total liabilities measured at fair value	0	0
Net assets measured at fair value	657,230	647,355
Fair Value, Inputs, Level 2 [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	2,390,931	2,386,754
Derivative financial instruments, net	5,611	3,070
Fair Value Liabilities, Convertible debentures - embedded derivative	0	0
Total liabilities measured at fair value	5,611	3,070
Net assets measured at fair value	2,385,320	2,383,684
Fair Value, Inputs, Level 3 [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	28,815	28,929
Derivative financial instruments, net	0	0
Fair Value Liabilities, Convertible debentures - embedded derivative	1,545	931
Total liabilities measured at fair value	1,545	931
Net assets measured at fair value	27,270	27,998
Cash And Cash Equivalents [Member] | Money Market Funds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	174,888	232,017
Cash And Cash Equivalents [Member] | Bank Certificates of Deposit [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	29,998	29,994
Cash And Cash Equivalents [Member] | Commercial Paper [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	159,983	233,980
Cash And Cash Equivalents [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	31,495	160,021
Cash And Cash Equivalents [Member] | Foreign Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	63,990	68,993
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	174,888	232,017
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Bank Certificates of Deposit [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	2,500	75,036
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] | Bank Certificates of Deposit [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	29,998	29,994
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	159,983	233,980
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	28,995	84,985
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	63,990	68,993
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Bank Certificates of Deposit [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Commercial Paper [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Cash And Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Bank Certificates of Deposit [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	159,916	129,978
Short-Term Investments [Member] | Commercial Paper [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	329,843	360,887
Short-Term Investments [Member] | Corporate Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	25,768	14,257
Short-Term Investments [Member] | Municipal Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	3,503
Short-Term Investments [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	532,899	442,694
Short-Term Investments [Member] | Foreign Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	164,958	180,958
Short-Term Investments [Member] | Mortgage-Backed Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	51	31
Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Bank Certificates of Deposit [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Municipal Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	434,860	322,763
Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Mortgage-Backed Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Bank Certificates of Deposit [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	159,916	129,978
Short-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	329,843	360,887
Short-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	25,768	14,257
Short-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Municipal Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	3,503
Short-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	98,039	119,931
Short-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	164,958	180,958
Short-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Mortgage-Backed Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	51	31
Short-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Bank Certificates of Deposit [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Commercial Paper [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Municipal Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Short-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Mortgage-Backed Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Corporate Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	185,104	175,415
Long-Term Investments [Member] | Auction Rate Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	28,815	28,929
Long-Term Investments [Member] | Municipal Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	16,711	26,160
Long-Term Investments [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	104,655	66,198
Long-Term Investments [Member] | Mortgage-Backed Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	1,024,159	892,745
Long-Term Investments [Member] | Debt Mutual Fund [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	40,240	19,781
Long-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Auction Rate Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Municipal Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	44,982	17,539
Long-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Mortgage-Backed Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Mutual Fund [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	185,104	175,415
Long-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Auction Rate Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Municipal Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	16,711	26,160
Long-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	59,673	48,659
Long-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Mortgage-Backed Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	1,024,159	892,745
Long-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Mutual Fund [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	40,240	19,781
Long-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Auction Rate Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	28,815	28,929
Long-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Municipal Bonds [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | U.S. Government and Agency Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Mortgage-Backed Securities [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0	0
Long-Term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Mutual Fund [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	$ 0	$ 0