Reclassification of Prior Periods (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Net Cash Provided by Operating Activities [Abstract]
Amortization of premium on available-for-sale investments	$ 10,754	$ 13,186	$ 15,520
Net cash provided by operating activities		459,365	411,978
Net Cash Provided by Used in Investing Activities [Abstract]
Purchases of available-for-sale investments	(1,337,482)	(998,977)	(1,149,145)
Net cash used in investing activities	(503,279)	(949,942)	(272,002)
Scenario, Previously Reported [Member]
Net Cash Provided by Operating Activities [Abstract]
Amortization of premium on available-for-sale investments		0	0
Net cash provided by operating activities		446,179	396,458
Net Cash Provided by Used in Investing Activities [Abstract]
Purchases of available-for-sale investments		(985,791)	(1,133,625)
Net cash used in investing activities		(936,756)	(256,482)
Restatement Adjustment [Member]
Net Cash Provided by Operating Activities [Abstract]
Amortization of premium on available-for-sale investments		13,186	15,520
Net cash provided by operating activities		13,186	15,520
Net Cash Provided by Used in Investing Activities [Abstract]
Purchases of available-for-sale investments		(13,186)	(15,520)
Net cash used in investing activities		$ (13,186)	$ (15,520)