2.125% Junior subordinated convertible debentures (Notes)	6 Months Ended
Sep. 30, 2012
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, 2012, none of the conditions allowing holders of the debentures to convert had been met. As a result of cash dividends paid since the issuance of the debentures, the conversion rate has been adjusted to 36.5491 shares of common stock per $1,000 of principal amount of debentures, representing a conversion price of approximately $27.36 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, 2012 and at March 31, 2012 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 which was further discounted due to embedded features as described below. The unamortized debt discount was $790.4 million at September 30, 2012 and $794.4 million at March 31, 2012. The carrying value of the debentures was $359.7 million at September 30, 2012 and $355.1 million at March 31, 2012. The remaining period over which the unamortized debt discount will be recognized as non-cash interest expense is 25.25 years. 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. In the three and six months ended September 30, 2011, the Company recognized $1.9 million and $3.7 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, 2012 and September 30, 2011, 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, 2012, none of the conditions allowing holders of the debentures to convert had been met. As a result of cash dividends paid since the issuance of the debentures, the conversion rate has been adjusted to 36.5491 shares of common stock per $1,000 of principal amount of debentures, representing a conversion price of approximately $27.36 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, 2012 and at March 31, 2012 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 which was further discounted due to embedded features as described below. The unamortized debt discount was $790.4 million at September 30, 2012 and $794.4 million at March 31, 2012. The carrying value of the debentures was $359.7 million at September 30, 2012 and $355.1 million at March 31, 2012. The remaining period over which the unamortized debt discount will be recognized as non-cash interest expense is 25.25 years. 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. In the three and six months ended September 30, 2011, the Company recognized $1.9 million and $3.7 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, 2012 and September 30, 2011, respectively.