Investments AFS (Details) (USD $)	Sep. 30, 2014	Mar. 31, 2014
Investments [Abstract]
Amount of short-term investments	$ 801,670,000	$ 878,182,000
Amount of long-term investments	841,277,000	798,712,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	833,300,000
Amount of corporate debt securities with no contractual maturity excluded from estimated fair value of available-for-sale securities	$ 6,200,000