Debt and Credit Facilities (Long-Term Debt Excluding Current Maturities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Long-term Debt, Excluding Current Maturities	$ 4,922.9		$ 3,740.7
6.625% Percent Senior Notes Due Two Thousand Twenty [Member]
Debt instrument, interest rate			2.625%
Long-term Debt, Excluding Current Maturities	$ 0.0		$ 299.4
Two Point Nine Percent Senior Notes Due Two Thousand Twenty One [Member] [Member]
Debt instrument, interest rate	2.90%		2.90%
Long-term Debt, Excluding Current Maturities	$ 299.1		$ 298.3	$ 300.0
9.00% Debentures Due 2021 [Member]
Debt instrument, interest rate	9.00%		9.00%
Long-term Debt, Excluding Current Maturities	$ 124.9		$ 124.9
4.250% Senior Notes Due 2013 [Member]
Debt instrument, interest rate	4.25%		4.25%
Long-term Debt, Excluding Current Maturities	$ 697.8		$ 697.1
7.20% Debentures Due 2014-2025 [Member]
Debt instrument, interest rate	7.20%		7.20%
Long-term Debt, Excluding Current Maturities	$ 37.3		$ 44.8
3.55% Senior Notes due 2024 [Member]
Debt instrument, interest rate	3.55%		3.55%
Long-term Debt, Excluding Current Maturities	$ 496.6		$ 495.9
6.48% Debentures Due 2025 [Member]
Debt instrument, interest rate	6.48%		6.48%
Long-term Debt, Excluding Current Maturities	$ 149.7		$ 149.7
Three Point Five Percent Senior notes Due Two Thousand Twenty Six [Member]
Debt instrument, interest rate	3.50%
Long-term Debt, Excluding Current Maturities	$ 396.8	$ 400.0	$ 0.0
Three Point Seven Five Percent Senior Notes Due Two Thousand Twenty Eight [Member]
Debt instrument, interest rate	3.75%		3.75%
Long-term Debt, Excluding Current Maturities	$ 545.1		$ 544.5	550.0
Three Point Eight Percent Senior Notes Due Two Thousand Twenty Nine [Member]
Debt instrument, interest rate	3.80%
Long-term Debt, Excluding Current Maturities	$ 743.6	750.0	$ 0.0
5.750% Senior Notes Due 2043 [Member]
Debt instrument, interest rate	5.75%		5.75%
Long-term Debt, Excluding Current Maturities	$ 494.5		$ 494.3
4.650% Percent Senior Notes due Twenty Forty Four [Member]
Debt instrument, interest rate	4.65%		4.65%
Long-term Debt, Excluding Current Maturities	$ 295.9		$ 295.8
Four Point Three Percent Senior Notes Due Two Thousand Forty Eight [Member]
Debt instrument, interest rate	4.30%		4.30%
Long-term Debt, Excluding Current Maturities	$ 296.0		$ 295.9	$ 300.0
Four Point Five Percent Senior Notes Due Two Thousand Forty Nine [Member]
Debt instrument, interest rate	4.50%
Long-term Debt, Excluding Current Maturities	$ 345.5	$ 350.0	$ 0.0
Other Loans And Notes [Member]
Debt instrument, interest rate	0.00%		0.00%
Long-term Debt, Excluding Current Maturities	$ 0.1		$ 0.1