2.125% Junior subordinated convertible debentures (Details) (USD $)	3 Months Ended	6 Months Ended			3 Months Ended		6 Months Ended		6 Months Ended
	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2013	Mar. 31, 2014	Sep. 30, 2014 Junior subordinated convertible debentures due 2037 [Member]	Sep. 30, 2013 Junior subordinated convertible debentures due 2037 [Member]	Sep. 30, 2014 Junior subordinated convertible debentures due 2037 [Member]	Mar. 31, 2014 Junior subordinated convertible debentures due 2037 [Member]	Sep. 30, 2014 Minimum [Member] Junior subordinated convertible debentures due 2037 [Member]	Sep. 30, 2014 Maximum [Member] Junior subordinated convertible debentures due 2037 [Member]
Debt Instrument [Line Items]
Principal amount of junior subordinated convertible debentures					$ 1,150,000,000.00		$ 1,150,000,000.00
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							39.2544
Debt Instrument, Convertible, Conversion Price					$ 25.47		$ 25.47
Long-term Debt, Contingent Payment of Interest, Percentage							0.50%		0.25%	0.50%
Debt Instrument, Convertible, Trading Price									$ 40	$ 150
Carrying value of equity component of the debentures that were bifurcated into a liability and equity component					822,400,000		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					772,400,000		772,400,000	777,200,000
Carrying value of debentures	376,729,000	376,729,000		371,873,000
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)							23 years 3 months
Amortization of Debt Discount (Premium)	2,400,000	4,810,000	4,396,000			2,200,000
Recognized amount of interest expense related to debentures					$ 6,100,000	$ 6,100,000	$ 12,200,000