Marketable securities (Details) (USD $)	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale debt securities: Estimated fair value	$ 2,050,518,000	$ 1,543,193,000
Total available-for-sale debt securities: Gains in accumulated other comprehensive income	7,206,000	2,306,000
Total available-for-sale debt securities: Losses in accumulated other comprehensive income	(2,966,000)	(10,800,000)
Current [Abstract]
Estimated fair value	739,198,000	847,555,000
Gains in accumulated other comprehensive income	1,445,000	1,186,000
Losses in accumulated other comprehensive income	(45,000)	(171,000)
Non-current [Abstract]
Estimated fair value	1,311,320,000	695,638,000
Gains in accumulated other comprehensive income	5,761,000	1,120,000
Losses in accumulated other comprehensive income	(2,921,000)	(10,629,000)
Proceeds from sales of available-for-sale debt securities	3,000,000,000	2,700,000,000
Gross realized gains on sales of available-for-sale debt securities	1,300,000	4,400,000
Net unrealized holding gains (losses) on available-for-sale debt securities	4,200,000	(8,500,000)
Investment in Ironwood Pharmaceuticals, Inc.	38,100,000	27,700,000
Contractual maturities of available-for-sale debt securities, fair value [Abstract]
Within one year	739,198,000
1-5 years	1,303,416,000
5-10 years	0
After 10 years	7,904,000
Total available-for-sale debt securities: Estimated fair value	2,050,518,000	1,543,193,000
Municipal Bonds and Notes [Member]
Current [Abstract]
Estimated fair value	34,025,000	33,723,000
Gains in accumulated other comprehensive income	34,000	52,000
Non-current [Abstract]
Estimated fair value	12,852,000	35,890,000
Gains in accumulated other comprehensive income	37,000	45,000
Government Agency Bonds [Member}
Current [Abstract]
Estimated fair value	87,227,000	92,829,000
Gains in accumulated other comprehensive income	125,000	123,000
Losses in accumulated other comprehensive income	(10,000)
Non-current [Abstract]
Estimated fair value	186,577,000	60,087,000
Gains in accumulated other comprehensive income	434,000	185,000
Losses in accumulated other comprehensive income	(19,000)
Commercial Paper [Member]
Current [Abstract]
Estimated fair value	144,293,000	239,393,000
Gains in accumulated other comprehensive income	0	334,000
Losses in accumulated other comprehensive income		(70,000)
Non-current [Abstract]
Estimated fair value		14,682,000
Gains in accumulated other comprehensive income		111,000
Certificates of Deposit [Member]
Current [Abstract]
Estimated fair value	47,977,000	91,819,000
Gains in accumulated other comprehensive income	0	320,000
Losses in accumulated other comprehensive income	(2,000)
Non-current [Abstract]
Estimated fair value	22,999,000
Gains in accumulated other comprehensive income	0
Corporate Bonds [Member]
Current [Abstract]
Estimated fair value	425,676,000	210,852,000
Gains in accumulated other comprehensive income	1,286,000	76,000
Losses in accumulated other comprehensive income	(33,000)	(79,000)
Non-current [Abstract]
Estimated fair value	1,084,194,000	305,697,000
Gains in accumulated other comprehensive income	5,290,000	779,000
Losses in accumulated other comprehensive income	(2,150,000)	(82,000)
Auction Rate Securities [Member]
Non-current [Abstract]
Estimated fair value	3,198,000	25,089,000
Losses in accumulated other comprehensive income	(752,000)
Variable Rate Note [Member]
Non-current [Abstract]
Estimated fair value	1,500,000
Losses in accumulated other comprehensive income	0
Floating rate notes [Member]
Current [Abstract]
Estimated fair value		178,939,000
Gains in accumulated other comprehensive income		281,000
Losses in accumulated other comprehensive income		(22,000)
Non-current [Abstract]
Estimated fair value		254,193,000
Losses in accumulated other comprehensive income		$ (10,547,000)