Investments AFS (Details) (USD $)	Sep. 30, 2013	Mar. 31, 2013
Investments [Abstract]
Amount of short-term investments	$ 807,709,000	$ 1,050,263,000
Amount of long-term investments	829,656,000	257,450,000
Amount of corporate debt securities with no contractual maturity excluded from estimated fair value of available-for-sale securities	$ 6,200,000