2.125% Junior subordinated convertible debentures (Details) (USD $)	3 Months Ended		6 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2012
Debt Instrument [Line Items]
Carrying value of debentures	$ 359,656,000		$ 359,656,000		$ 355,050,000
Amortization of Debt Discount (Premium)	2,000,000	1,900,000	4,017,000	3,671,000
Junior subordinated convertible debentures due 2037 [Member]
Debt Instrument [Line Items]
Principal amount of junior subordinated convertible debentures	1,150,000,000		1,150,000,000
Interest rate of junior subordinated convertible debentures	2.125%		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		$ 34.16
Debt Instrument, Convertible, Conversion Ratio			36.5491
Debt Instrument, Convertible, Conversion Price	$ 27.36		$ 27.36
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		327,600,000
Debt discount of estimated fair value of liability component of debentures	822,400,000		822,400,000
Amount of unamortized debt discount of debentures	790,400,000		790,400,000		794,400,000
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)	25.3		25.3
Recognized amount of interest expense related to debentures	$ 6,100,000	$ 6,100,000	$ 12,200,000	$ 12,200,000