2.125% Junior subordinated convertible debentures (Details) (USD $)	3 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Mar. 31, 2012
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
Convertible Debt Instrument Initial Conversion Price Per Share	$ 34.16
Debt Instrument, Convertible, Conversion Ratio	36.1901
Debt Instrument, Convertible, Conversion Price	$ 27.63
Carrying value of equity component of the debentures that were bifurcated into a liability and equity component	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	792,400,000		794,400,000
Carrying value of debentures	357,355,000		355,050,000
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)	25.5
Amortization of Debt Discount (Premium)	1,975,000	1,805,000
Recognized amount of interest expense related to debentures	$ 6,100,000	$ 6,100,000