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Fair Value Measurements (Details Textual) (USD $)	3 Months Ended		1 Months Ended						3 Months Ended
Fair Value Measurements (Details Textual) (USD $)	Jun. 30, 2012	Mar. 31, 2012	Mar. 31, 2007 3.125% Junior Convertible Debentures [Member]	Jun. 30, 2012 3.125% Junior Convertible Debentures [Member]	Mar. 31, 2012 3.125% Junior Convertible Debentures [Member]	Jun. 30, 2012 2.625% Senior Convertible Debentures [Member]	Mar. 31, 2012 2.625% Senior Convertible Debentures [Member]	Jun. 30, 2010 2.625% Senior Convertible Debentures [Member]	Jun. 30, 2012 Minimum [Member]	Jun. 30, 2012 Maximum [Member]	Jun. 30, 2012 Auction Rate Securities [Member]	Mar. 31, 2012 Auction Rate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 3,076,976,000	$ 3,063,038,000									$ 28,815,000	$ 28,929,000
Weighted Average Life For Cash Flow Projected On Fair Value Of Student Loan Auction Rate Securities	8 years
Discount Rate On Fair Value Of Student Loan Auction Rate Securities Range									2.51%	3.23%
Hypothetical increase or decrease percentage of the weighted average life assumption	20.00%
Hypothetical change in percentage input to valuation model of level 3 assets	1.00%
Student loan auction rate securities maturity date									12/31/2027	5/31/2046
Debt Instruments [Abstract]
Principal amount of convertible debentures			1,000,000,000	689,635,000	689,635,000	600,000,000	600,000,000	600,000,000
Interest rate of debentures	3.125%		3.125%			2.625%
Convertible debentures due date			Mar. 15, 2037
Fair value of debentures				$ 826,100,000		$ 766,500,000