Summary of Available for Sale (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,677,422	$ 1,301,989
Available-for-sale, gross unrealized gains	3,462	6,189
Available-for-sale, gross unrealized losses	(3,990)	(465)
Available-for-sale, estimated fair value	1,676,894	1,307,713
Government agency bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	684,451	558,116
Available-for-sale, gross unrealized gains	114	335
Available-for-sale, gross unrealized losses	(3,171)	(298)
Available-for-sale, estimated fair value	681,394	558,153
Municipal bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	41,622	25,000
Available-for-sale, gross unrealized gains	101	146
Available-for-sale, gross unrealized losses	(14)	(8)
Available-for-sale, estimated fair value	41,709	25,138
Auction rate securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	9,825	33,459
Available-for-sale, gross unrealized gains	0	332
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	9,825	33,791
Corporate bonds and debt [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	941,524	680,144
Available-for-sale, gross unrealized gains	3,247	5,137
Available-for-sale, gross unrealized losses	(805)	(159)
Available-for-sale, estimated fair value	943,966	685,122
Equity Securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost		5,270
Available-for-sale, gross unrealized gains		239
Available-for-sale, gross unrealized losses		0
Available-for-sale, estimated fair value		$ 5,509