Marketable securities (Details) (USD $)	6 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale debt securities: Estimated fair value	$ 1,760,554,000		$ 1,543,193,000
Total available-for-sale debt securities: Gains in accumulated other comprehensive income	6,496,000		2,306,000
Total available-for-sale debt securities: Losses in accumulated other comprehensive income	(14,064,000)		(10,800,000)
Current [Abstract]
Estimated fair value	818,621,000		847,555,000
Gains in accumulated other comprehensive income	1,241,000		1,186,000
Losses in accumulated other comprehensive income	(5,895,000)		(171,000)
Non-current [Abstract]
Estimated fair value	941,933,000		695,638,000
Gains in accumulated other comprehensive income	5,255,000		1,120,000
Losses in accumulated other comprehensive income	(8,169,000)		(10,629,000)
Proceeds from sales of available-for-sale debt securities	1,100,000,000	1,700,000,000
Gross realized gains on sales of available-for-sale debt securities	1,000,000	2,300,000
Net unrealized holding losses on available-for-sale debt securities	7,600,000		8,500,000
Investment in Ironwood Pharmaceuticals, Inc.	26,600,000		27,700,000
Contractual maturities of available-for-sale debt securities, fair value [Abstract]
Within one year	818,621,000
1-5 years	909,894,000
5-10 years	20,184,000
After 10 years	11,855,000
Total available-for-sale debt securities: Estimated fair value	1,760,554,000		1,543,193,000
Municipal Bonds and Notes [Member]
Current [Abstract]
Estimated fair value	45,067,000		33,723,000
Gains in accumulated other comprehensive income			52,000
Losses in accumulated other comprehensive income	(1,839,000)
Non-current [Abstract]
Estimated fair value	22,996,000		35,890,000
Gains in accumulated other comprehensive income	33,000		45,000
Losses in accumulated other comprehensive income	(319,000)
Government Agency Bonds [Member}
Current [Abstract]
Estimated fair value	63,992,000		92,829,000
Gains in accumulated other comprehensive income			123,000
Losses in accumulated other comprehensive income	(1,096,000)
Non-current [Abstract]
Estimated fair value	83,205,000		60,087,000
Gains in accumulated other comprehensive income	109,000		185,000
Losses in accumulated other comprehensive income	(56,000)
Commercial Paper [Member]
Current [Abstract]
Estimated fair value	228,051,000		239,393,000
Gains in accumulated other comprehensive income	648,000		334,000
Losses in accumulated other comprehensive income			(70,000)
Non-current [Abstract]
Estimated fair value	16,615,000		14,682,000
Gains in accumulated other comprehensive income	101,000		111,000
Certificates of Deposit [Member]
Current [Abstract]
Estimated fair value	87,522,000		91,819,000
Gains in accumulated other comprehensive income	156,000		320,000
Losses in accumulated other comprehensive income	(3,000)
Non-current [Abstract]
Estimated fair value	7,994,000
Losses in accumulated other comprehensive income	(5,000)
Corporate Bonds [Member]
Current [Abstract]
Estimated fair value	302,902,000		210,852,000
Gains in accumulated other comprehensive income	105,000		76,000
Losses in accumulated other comprehensive income	(2,957,000)		(79,000)
Non-current [Abstract]
Estimated fair value	676,624,000		305,697,000
Gains in accumulated other comprehensive income	2,618,000		779,000
Losses in accumulated other comprehensive income	(16,000)		(82,000)
Auction Rate Securities [Member]
Non-current [Abstract]
Estimated fair value	14,773,000		25,089,000
Losses in accumulated other comprehensive income	(752,000)
Floating Rate Notes [Member]
Current [Abstract]
Estimated fair value	91,087,000		178,939,000
Gains in accumulated other comprehensive income	332,000		281,000
Losses in accumulated other comprehensive income			(22,000)
Non-current [Abstract]
Estimated fair value	119,726,000		254,193,000
Gains in accumulated other comprehensive income	2,394,000
Losses in accumulated other comprehensive income	$ (7,021,000)		$ (10,547,000)