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Debt - Long-term (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019
Debt Instrument [Line Items]
Debt issuance cost	$ (19.4)	$ (20.2)
Total long-term debt	3,595.7	3,113.9
Less: debt maturing within one year	63.9	82.8
Total long-term debt, less current portion	3,531.8	3,031.1
Pollution Control and Industrial Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term debt, gross	51.7	51.6
Unamortized discount	$ 0.1		$ 0.2
Pollution Control and Industrial Revenue Bonds [Member] \| Minimum
Debt Instrument [Line Items]
Interest Rate Percentage	4.00%
Pollution Control and Industrial Revenue Bonds [Member] \| Maximum
Debt Instrument [Line Items]
Interest Rate Percentage	6.50%
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt, gross	$ 2,198.8	2,198.7
Unamortized discount	$ 1.2		$ 1.3
Senior Notes [Member] \| Minimum
Debt Instrument [Line Items]
Interest Rate Percentage	3.20%
Senior Notes [Member] \| Maximum
Debt Instrument [Line Items]
Interest Rate Percentage	4.50%
2017 Term Loan Facility
Debt Instrument [Line Items]
Interest Rate Percentage	2.20%
Long-term debt, gross	$ 800.0	800.0
Revolving Credit Facility
Debt Instrument [Line Items]
Interest Rate Percentage	1.90%
Long-term debt, gross	$ 500.0	0.0
Letters of credit outstanding amount	215.1
Credit agreement, available funds	784.9
Foreign debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 64.6	$ 83.8
Foreign debt \| Minimum
Debt Instrument [Line Items]
Interest Rate Percentage	0.00%
Foreign debt \| Maximum
Debt Instrument [Line Items]
Interest Rate Percentage	7.20%