Investments AFS (Details) (USD $)	3 Months Ended		9 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2011
Notes to Financial Statements [Abstract]
Amount of short-term investments	$ 696,548,000		$ 696,548,000		$ 539,572,000
Amount of long-term investments	468,086,000		468,086,000		464,838,000
Impairment amount in value of auction rate securities failed	0	400,000	800,000	1,300,000
Amount of corporate debt securities with no contractual maturity excluded from estimated fair value of available-for-sale securities	4,600,000		4,600,000
Amount of net realized gains (losses) on sales of available-for-sale marketable equity securities	100,000		200,000
Amount of net realized gains (losses) on sales of available-for-sale debt securities	100,000	100,000	100,000	(100,000)
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	$ 200,000