Investments AFS (Details) (USD $)	3 Months Ended		6 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Mar. 31, 2011
Notes to Financial Statements [Abstract]
Amount of short-term investments	$ 740,309,000		$ 740,309,000		$ 539,572,000
Amount of long-term investments	359,980,000		359,980,000		464,838,000
Amount of investment in auction rate securities	11,700,000		11,700,000
Impairment amount in value of auction rate securities failed	400,000	400,000	800,000	800,000
Amount of net realized gains (losses) on sales of available-for-sale securities	(900,000)		100,000	200,000
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