Fair Value Measurements - Financial Assets and Liabilities that are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Sep. 30, 2017 | Mar. 31, 2017 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | $ 80.5 | $ 116.3 | $ 92.6 |
Derivative financial instruments, liabilities | (85.4) | (82.5) | (42.7) |
Fair Value, Measurements, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Non-qualified supplemental postretirement grantor trust investments | 37.9 | 35.6 | 35.2 |
Fair Value, Measurements, Recurring | Commodity contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 66.6 | 104.1 | 74 |
Derivative financial instruments, liabilities | (37.8) | (39.1) | (39) |
Fair Value, Measurements, Recurring | Foreign currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 13.9 | 12.2 | 15.6 |
Derivative financial instruments, liabilities | (43.5) | (38.2) | (1.5) |
Fair Value, Measurements, Recurring | Interest rate contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, liabilities | (1.9) | (2.3) | (2.2) |
Fair Value, Measurements, Recurring | Cross-currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | | | 3 |
Derivative financial instruments, liabilities | (2.2) | (2.9) | |
Fair Value, Measurements, Recurring | Level 1 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Non-qualified supplemental postretirement grantor trust investments | 37.9 | 35.6 | 35.2 |
Fair Value, Measurements, Recurring | Level 1 | Commodity contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 39.6 | 27.2 | 56 |
Derivative financial instruments, liabilities | (24.3) | (27.7) | (28) |
Fair Value, Measurements, Recurring | Level 1 | Foreign currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 0 | 0 | 0 |
Derivative financial instruments, liabilities | 0 | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Interest rate contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, liabilities | 0 | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Cross-currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | | | 0 |
Derivative financial instruments, liabilities | 0 | 0 | |
Fair Value, Measurements, Recurring | Level 2 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Non-qualified supplemental postretirement grantor trust investments | 0 | 0 | 0 |
Fair Value, Measurements, Recurring | Level 2 | Commodity contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 27 | 76.9 | 18 |
Derivative financial instruments, liabilities | (13.5) | (11.4) | (11) |
Fair Value, Measurements, Recurring | Level 2 | Foreign currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 13.9 | 12.2 | 15.6 |
Derivative financial instruments, liabilities | (43.5) | (38.2) | (1.5) |
Fair Value, Measurements, Recurring | Level 2 | Interest rate contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, liabilities | (1.9) | (2.3) | (2.2) |
Fair Value, Measurements, Recurring | Level 2 | Cross-currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | | | 3 |
Derivative financial instruments, liabilities | (2.2) | (2.9) | |
Fair Value, Measurements, Recurring | Level 3 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Non-qualified supplemental postretirement grantor trust investments | 0 | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Commodity contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 0 | 0 | 0 |
Derivative financial instruments, liabilities | 0 | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Foreign currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 0 | 0 | 0 |
Derivative financial instruments, liabilities | 0 | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Interest rate contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, liabilities | 0 | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Cross-currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | | | $ 0 |
Derivative financial instruments, liabilities | $ 0 | $ 0 | |