v2.4.0.8
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