Junior subordinated convertible debentures (Details) (USD $)	12 Months Ended
	Mar. 31, 2011	Mar. 31, 2010	Mar. 31, 2009
Debt Instrument [Line Items]
Principal amount of junior subordinated convertible debentures	$ 1,150,000,000
Interest rate of junior subordinated convertible debentures	2.125%
Due date of junior subordinated convertible debentures	Dec. 15, 2037
Conversion rate, in shares, of junior subordinated convertible debt (in shares per $1,000)	29.2783
Principal amount of junior subordinated convertible debentures used as conversion unit	1,000
Approximate initial conversion price, per share, of junior subordinated convertible debentures (in dollars per share)	34.16
Adjusted conversion rate, in shares, of junior subordinated convertible debt (in shares per $1,000)	34.4383
Approximate adjusted conversion price, per share, of junior subordinated convertible debentures due to dividends paid (in dollars per share)	29.04
Carrying value of equity component of the debentures that were bifurcated into a liability and equity component	822,400,000	822,400,000
Estimated fair value of liability component of debentures at issuance date	327,600,000
Debt discount of estimated fair value of liability component of debentures	822,400,000
Amount of unamortized debt discount of debentures	801,900,000	808,700,000
Carrying value of debentures	347,334,000	340,672,000
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)	26.75
Amount of non-cash interest expense related to the amortization of debt discount	6,800,000	6,300,000	5,200,000
Recognized amount of interest expense related to debentures	24,400,000	24,400,000	24,400,000
Fair value of derivative on certain embedded features of debentures	400,000	700,000
Amount of decrease in interest expense from change in the amount of fair value of derivative	$ 200,000