Investments AFS (Details) (USD $)	3 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Mar. 31, 2012
Investments [Abstract]
Amount of short-term investments	$ 881,913,000		$ 823,254,000
Amount of long-term investments	159,476,000		328,586,000
Amount of corporate debt securities with no contractual maturity excluded from estimated fair value of available-for-sale securities	4,600,000
Amount of net realized gains (losses) on sales of available-for-sale marketable equity securities	100,000	1,000,000
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount		$ 1,000,000