Debt and Financing - Short and Long Term Debt (Details) - USD ($)	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Mar. 31, 2011
Debt Instrument [Line Items]
Debt, gross	$ 1,950,000,000
Unaccreted Discount	(1,300,000)
Total debt	1,948,700,000
Short-term debt	299,900,000		$ 0
Long-term debt	1,648,800,000		$ 1,349,000,000
Fixed Rate Note Due 2016 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 300,000,000				$ 300,000,000
Stated interest rate	3.10%				3.10%
Effective interest rate	3.25%
Fixed rate note due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 300,000,000	$ 300,000,000
Stated interest rate	3.30%	3.30%
Effective interest rate	3.47%
Fixed Rate Note Due 2021 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 300,000,000				$ 300,000,000
Stated interest rate	4.60%				4.60%
Effective interest rate	4.69%
Fixed Rate Note Due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 350,000,000			$ 350,000,000
Stated interest rate	4.50%			4.50%
Effective interest rate	4.63%
Fixed Rate Note Due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 300,000,000	$ 300,000,000
Stated interest rate	4.35%	4.35%
Effective interest rate	4.47%
Fixed Rate Note Due 2041 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 400,000,000				$ 400,000,000
Stated interest rate	5.95%				5.95%
Effective interest rate	6.03%