Summary of Available for Sale (Details) (USD $)	6 Months Ended	12 Months Ended
	Sep. 30, 2014	Mar. 31, 2014
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,643,964,000	$ 1,677,422,000
Available-for-sale, gross unrealized gains	2,345,000	3,462,000
Available-for-sale, gross unrealized losses	(3,362,000)	(3,990,000)
Available-for-sale, estimated fair value	1,642,947,000	1,676,894,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position	833,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position	(3,362,000)
Government agency bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	668,639,000	684,451,000
Available-for-sale, gross unrealized gains	129,000	114,000
Available-for-sale, gross unrealized losses	(2,560,000)	(3,171,000)
Available-for-sale, estimated fair value	666,208,000	681,394,000
Municipal bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	41,097,000	41,622,000
Available-for-sale, gross unrealized gains	66,000	101,000
Available-for-sale, gross unrealized losses	(2,000)	(14,000)
Available-for-sale, estimated fair value	41,161,000	41,709,000
Auction rate securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	9,825,000	9,825,000
Available-for-sale, gross unrealized gains	0	0
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	9,825,000	9,825,000
Time Deposits [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	20,160,000
Available-for-sale, gross unrealized gains	0
Available-for-sale, gross unrealized losses	0
Available-for-sale, estimated fair value	20,160,000
Corporate bonds and debt [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	904,243,000	941,524,000
Available-for-sale, gross unrealized gains	2,150,000	3,247,000
Available-for-sale, gross unrealized losses	(800,000)	(805,000)
Available-for-sale, estimated fair value	$ 905,593,000	$ 943,966,000