2.125% Junior subordinated convertible debentures (Notes)	6 Months Ended
Sep. 30, 2013
Convertible Debt [Abstract]
Junior subordinated convertible debentures	2.125% Junior Subordinated Convertible Debentures The Company's $1.15 billion principal amount of 2.125% junior subordinated convertible debentures due December 15, 2037, are subordinated in right of payment to any future senior debt of the Company and are effectively subordinated in right of payment to the liabilities of the Company's subsidiaries. The debentures are convertible, subject to certain conditions, into shares of the Company's common stock at an initial conversion rate of 29.2783 shares of common stock per $1,000 principal amount of debentures, representing an initial conversion price of approximately $34.16 per share of common stock. As of September 30, 2013, the holders of the debentures had the right to convert their debentures between July 1, 2013 and September 30, 2013 because for at least 20 trading days during the 30 consecutive trading day period ending on September 30, 2013, the Company's common stock had a last reported sale price greater than 130% of the conversion price. As of September 30, 2013, a holder could realize more economic value by selling its debentures in the over the counter market than from converting its debentures. As a result of cash dividends paid since the issuance of the debentures, the conversion rate has been adjusted to 38.0436 shares of common stock per $1,000 of principal amount of debentures, representing a conversion price of approximately $26.29 per share of common stock. As the debentures can be settled in cash upon conversion, for accounting purposes, the debentures were bifurcated into a liability component and an equity component, which are both initially recorded at fair value. The carrying value of the equity component at September 30, 2013 and at March 31, 2013 was $822.4 million. The estimated fair value of the liability component of the debentures at the issuance date was $327.6 million, resulting in a debt discount of $822.4 million. The unamortized debt discount was $781.8 million at September 30, 2013 and $786.2 million at March 31, 2013. The carrying value of the debentures was $367.5 million at September 30, 2013 and $363.4 million at March 31, 2013. The remaining period over which the unamortized debt discount will be recognized as non-cash interest expense is 24.25 years. In the three and six months ended September 30, 2013, the Company recognized $2.2 million and $4.4 million, respectively, in non-cash interest expense related to the amortization of the debt discount. In the three and six months ended September 30, 2012, the Company recognized $2.0 million and $4.0 million, respectively, in non-cash interest expense related to the amortization of the debt discount. The Company recognized $6.1 million and $12.2 million of interest expense related to the 2.125% coupon on the debentures in each of the three and six-month periods ended September 30, 2013 and September 30, 2012, respectively.

2.125% Junior Subordinated Convertible Debentures

The Company's $1.15 billion principal amount of 2.125% junior subordinated convertible debentures due December 15, 2037, are subordinated in right of payment to any future senior debt of the Company and are effectively subordinated in right of payment to the liabilities of the Company's subsidiaries. The debentures are convertible, subject to certain conditions, into shares of the Company's common stock at an initial conversion rate of 29.2783 shares of common stock per $1,000 principal amount of debentures, representing an initial conversion price of approximately $34.16 per share of common stock. As of September 30, 2013, the holders of the debentures had the right to convert their debentures between July 1, 2013 and September 30, 2013 because for at least 20 trading days during the 30 consecutive trading day period ending on September 30, 2013, the Company's common stock had a last reported sale price greater than 130% of the conversion price. As of September 30, 2013, a holder could realize more economic value by selling its debentures in the over the counter market than from converting its debentures. As a result of cash dividends paid since the issuance of the debentures, the conversion rate has been adjusted to 38.0436 shares of common stock per $1,000 of principal amount of debentures, representing a conversion price of approximately $26.29 per share of common stock.

As the debentures can be settled in cash upon conversion, for accounting purposes, the debentures were bifurcated into a liability component and an equity component, which are both initially recorded at fair value. The carrying value of the equity component at September 30, 2013 and at March 31, 2013 was $822.4 million. The estimated fair value of the liability component of the debentures at the issuance date was $327.6 million, resulting in a debt discount of $822.4 million. The unamortized debt discount was $781.8 million at September 30, 2013 and $786.2 million at March 31, 2013. The carrying value of the debentures was $367.5 million at September 30, 2013 and $363.4 million at March 31, 2013. The remaining period over which the unamortized debt discount will be recognized as non-cash interest expense is 24.25 years. In the three and six months ended September 30, 2013, the Company recognized $2.2 million and $4.4 million, respectively, in non-cash interest expense related to the amortization of the debt discount. In the three and six months ended September 30, 2012, the Company recognized $2.0 million and $4.0 million, respectively, in non-cash interest expense related to the amortization of the debt discount. The Company recognized $6.1 million and $12.2 million of interest expense related to the 2.125% coupon on the debentures in each of the three and six-month periods ended September 30, 2013 and September 30, 2012, respectively.