2.125% Junior subordinated convertible debentures (Details)	3 Months Ended		6 Months Ended
	Sep. 30, 2016 USD ($) $ / shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) day $ / shares	Sep. 30, 2015 USD ($)	Mar. 31, 2016 USD ($)
Debt Instrument [Line Items]
Amortization of debt discount on convertible debentures			$ 24,413,000	$ 23,746,000
Junior Subordinated Convertible Debentures Due 2037
Debt Instrument [Line Items]
Principal amount of junior subordinated convertible debentures	$ 575,000,000.0		$ 575,000,000.0
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 | $ / shares	$ 34.16		$ 34.16
Debt instrument, convertible, threshold trading days | day			20
Debt instrument, convertible, threshold consecutive trading days			30 days
Debt instrument, convertible, threshold percentage of stock price trigger			130.00%
Debt instrument, convertible, conversion ratio			41.6895
Debt instrument, convertible, conversion price | $ / shares	$ 23.99		$ 23.99
Debt instrument, convertible, if-converted value in excess of principal			$ 914,600,000
Long-term debt, contingent payment of interest, percentage			0.50%
Carrying value of equity component of the debentures that were bifurcated into a liability and equity component	$ 411,200,000		$ 411,200,000		$ 411,200,000
Estimated fair value of liability component of debentures at issuance date	163,800,000		163,800,000
Debt discount of estimated fair value of liability component of debentures	411,200,000		411,200,000
Amount of unamortized debt discount of debentures	375,400,000		$ 375,400,000		$ 378,300,000
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)			21 years 2 months 15 days
Amortization of debt discount on convertible debentures	1,500,000	$ 1,300,000	$ 2,900,000	2,600,000
Recognized amount of interest expense related to debentures	$ 3,100,000	$ 3,100,000	$ 6,200,000	$ 6,100,000
Effective interest rate of junior subordinated convertible debentures			9.10%
Junior Subordinated Convertible Debentures Due 2037 | Minimum
Debt Instrument [Line Items]
Long-term debt, contingent payment of interest, percentage			0.25%
Debt instrument, convertible, trading price | $ / shares	$ 400		$ 400
Junior Subordinated Convertible Debentures Due 2037 | Maximum
Debt Instrument [Line Items]
Long-term debt, contingent payment of interest, percentage			0.50%
Debt instrument, convertible, trading price | $ / shares	$ 1,500		$ 1,500