Summary of Available for Sale (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Jun. 30, 2014	Mar. 31, 2014
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,667,120	$ 1,677,422
Available-for-sale, gross unrealized gains	3,994	3,462
Available-for-sale, gross unrealized losses	(1,484)	(3,990)
Available-for-sale, estimated fair value	1,669,630	1,676,894
Government agency bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	684,090	684,451
Available-for-sale, gross unrealized gains	318	114
Available-for-sale, gross unrealized losses	(1,108)	(3,171)
Available-for-sale, estimated fair value	683,300	681,394
Municipal bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	41,347	41,622
Available-for-sale, gross unrealized gains	225	101
Available-for-sale, gross unrealized losses	(7)	(14)
Available-for-sale, estimated fair value	41,565	41,709
Auction rate securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	9,825	9,825
Available-for-sale, gross unrealized gains	0	0
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	9,825	9,825
Corporate bonds and debt [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	931,858	941,524
Available-for-sale, gross unrealized gains	3,451	3,247
Available-for-sale, gross unrealized losses	(369)	(805)
Available-for-sale, estimated fair value	$ 934,940	$ 943,966