Investments AFS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Mar. 31, 2015
Investments [Abstract]
Amount of short-term investments	$ 1,462,466		$ 1,462,466		$ 1,351,054
Amount of long-term investments	584,910		584,910		$ 383,326
Proceeds for sale of available-for-sale securities	$ 46,700	$ 26,700	135,800	$ 170,700
Realized gain on available-for-sale securities			13,959	$ 0
Amount of corporate debt securities with no contractual maturity excluded from estimated fair value of available-for-sale securities	$ 6,200		$ 6,200