Fair value measurements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Unobservable Market Inputs (Level 3) [Member]	Mar. 31, 2013 Recurring Basis [Member]	Mar. 31, 2012 Recurring Basis [Member]	Mar. 31, 2013 Recurring Basis [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Mar. 31, 2012 Recurring Basis [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Mar. 31, 2013 Recurring Basis [Member] Significant Other Observable Market Inputs (Level 2) [Member]	Mar. 31, 2012 Recurring Basis [Member] Significant Other Observable Market Inputs (Level 2) [Member]	Mar. 31, 2013 Recurring Basis [Member] Unobservable Market Inputs (Level 3) [Member]	Mar. 31, 2012 Recurring Basis [Member] Unobservable Market Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money market accounts			$ 818,474	$ 1,059,868	$ 818,474	$ 938,526		$ 121,342
Municipal bonds and notes			46,877	69,613			46,877	69,613
Commercial paper			168,639	556,794	31,815	284,981	136,824	271,813
Variable rate demand notes			1,500	4,000			1,500	4,000
Floating rate notes				467,259		467,259
Auction rate securities			3,198	25,089					3,198	25,089
Certificates of deposit			90,268	215,801	5,981	87,904	84,287	127,897
Corporate bonds			1,509,870	568,775			1,509,870	568,775
Government agency bonds			278,804	152,916			278,804	152,916
Reconciliation of Level 3 investments measured at fair value on a recurring basis using unobservable inputs [Roll Forward]
Balance at beginning of period	25,089
Sales	(21,139)	(21,100)
Unrealized loss	(752)
Balance at end of period	3,198
Sales of level 3 investments	$ 21,139	$ 21,100