Financial Instruments (Details 1) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	$ 769,780	$ 731,520
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(2,546)	(1,617)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	48,793	45,218
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(3,708)	(3,795)
Available-for-Sale Securities, Fair Value, Total	818,573	776,738
Available-for-Sale Securities, Gross Unrealized Losses, Total	(6,254)	(5,412)
Commercial Paper [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value		79,994
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses		(1)
Available-for-Sale Securities, 12 Months or Greater, Fair Value		0
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses		0
Available-for-Sale Securities, Fair Value, Total		79,994
Available-for-Sale Securities, Gross Unrealized Losses, Total		(1)
Corporate Bonds [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	30,040	21,111
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(122)	(184)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	1,782	0
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(92)	0
Available-for-Sale Securities, Fair Value, Total	31,822	21,111
Available-for-Sale Securities, Gross Unrealized Losses, Total	(214)	(184)
Auction Rate Securities [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	0	0
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	0	0
Available-for-Sale Securities, 12 Months or Greater, Fair Value	28,814	28,929
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(3,436)	(3,671)
Available-for-Sale Securities, Fair Value, Total	28,814	28,929
Available-for-Sale Securities, Gross Unrealized Losses, Total	(3,436)	(3,671)
Municipal Bonds [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	2,271	2,173
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(56)	(24)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	717	366
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(5)	(4)
Available-for-Sale Securities, Fair Value, Total	2,988	2,539
Available-for-Sale Securities, Gross Unrealized Losses, Total	(61)	(28)
U.S. Government and Agency Securities [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	472,979	460,735
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(92)	(149)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	0	0
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	0	0
Available-for-Sale Securities, Fair Value, Total	472,979	460,735
Available-for-Sale Securities, Gross Unrealized Losses, Total	(92)	(149)
Mortgage-Backed Securities [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	224,250	147,726
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(2,016)	(1,040)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	17,480	15,923
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(175)	(120)
Available-for-Sale Securities, Fair Value, Total	241,730	163,649
Available-for-Sale Securities, Gross Unrealized Losses, Total	(2,191)	(1,160)
Debt Mutual Fund [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	40,240	19,781
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(260)	(219)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	0	0
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	0	0
Available-for-Sale Securities, Fair Value, Total	40,240	19,781
Available-for-Sale Securities, Gross Unrealized Losses, Total	$ (260)	$ (219)