Marketable securities (Details) (USD $)	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale debt securities: Estimated fair value	$ 1,543,193,000	$ 2,214,095,000
Total available-for-sale debt securities: Gains in accumulated other comprehensive income	2,306,000	6,433,000
Total available-for-sale debt securities: Losses in accumulated other comprehensive income	(10,800,000)	(18,972,000)
Current [Abstract]
Estimated fair value	847,555,000	1,713,303,000
Gains in accumulated other comprehensive income	1,186,000	1,517,000
Losses in accumulated other comprehensive income	(171,000)	(11,760,000)
Noncurrent [Abstract]
Estimated fair value	695,638,000	500,792,000
Gains in accumulated other comprehensive income	1,120,000	4,916,000
Losses in accumulated other comprehensive income	(10,629,000)	(7,212,000)
Proceeds from sales of available-for-sale debt securities	2,700,000,000	2,900,000,000
Gross realized gains on sales of available-for-sale debt securities	4,400,000	9,300,000
Net unrealized holding losses on available-for-sale debt securities	8,500,000	12,500,000
Investment in Ironwood Pharmaceuticals, Inc.	27,700,000	29,100,000
Contractual maturities of available-for-sale debt securities, fair value [Abstract]
Within one year	847,555,000
1-5 years	659,786,000
5-10 years	15,196,000
After 10 years	20,656,000
Total available-for-sale debt securities	1,543,193,000	2,214,095,000
Variable Rate Demand Note [Member]
Current [Abstract]
Estimated fair value		178,435,000
Municipal bonds and notes [Member]
Current [Abstract]
Estimated fair value	33,723,000	144,950,000
Gains in accumulated other comprehensive income	52,000	195,000
Noncurrent [Abstract]
Estimated fair value	35,890,000	13,534,000
Gains in accumulated other comprehensive income	45,000	21,000
Government Agencies Debt Securities [Member]
Current [Abstract]
Estimated fair value	92,829,000	160,894,000
Gains in accumulated other comprehensive income	123,000	207,000
Noncurrent [Abstract]
Estimated fair value	60,087,000	54,598,000
Gains in accumulated other comprehensive income	185,000	4,504,000
Losses in accumulated other comprehensive income		(122,000)
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Current [Abstract]
Estimated fair value	239,393,000	606,986,000
Gains in accumulated other comprehensive income	334,000	753,000
Losses in accumulated other comprehensive income	(70,000)	(107,000)
Noncurrent [Abstract]
Estimated fair value	14,682,000
Gains in accumulated other comprehensive income	111,000
Certificate of deposits [Member]
Current [Abstract]
Estimated fair value	91,819,000	241,964,000
Gains in accumulated other comprehensive income	320,000	73,000
Noncurrent [Abstract]
Estimated fair value		9,436,000
Losses in accumulated other comprehensive income		(1,000)
Corporate Debt Securities [Member]
Current [Abstract]
Estimated fair value	210,852,000	252,146,000
Gains in accumulated other comprehensive income	76,000	289,000
Losses in accumulated other comprehensive income	(79,000)	(71,000)
Noncurrent [Abstract]
Estimated fair value	305,697,000	266,366,000
Gains in accumulated other comprehensive income	779,000
Losses in accumulated other comprehensive income	(82,000)	(2,401,000)
Auction Rate Securities [Member]
Noncurrent [Abstract]
Estimated fair value	25,089,000	34,539,000
Losses in accumulated other comprehensive income		(1,906,000)
Floating rate notes [Member]
Current [Abstract]
Estimated fair value	178,939,000	127,928,000
Gains in accumulated other comprehensive income	281,000
Losses in accumulated other comprehensive income	(22,000)	(11,582,000)
Noncurrent [Abstract]
Estimated fair value	254,193,000	122,319,000
Gains in accumulated other comprehensive income		391,000
Losses in accumulated other comprehensive income	$ (10,547,000)	$ (2,782,000)