Fair Value Measurements - Financial Assets and Liabilities that are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Sep. 30, 2017 | Jun. 30, 2017 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | $ 147.6 | $ 116.3 | $ 51.1 |
Derivative financial instruments, liabilities | (53.6) | (82.5) | (70.3) |
Fair Value, Measurements, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Non-qualified supplemental postretirement grantor trust investments | 38.4 | 35.6 | 35.8 |
Fair Value, Measurements, Recurring | Commodity contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 128.5 | 104.1 | 39.8 |
Derivative financial instruments, liabilities | (34.2) | (39.1) | (42.9) |
Fair Value, Measurements, Recurring | Foreign currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 18.4 | 12.2 | 11.3 |
Derivative financial instruments, liabilities | (17.9) | (38.2) | (24.3) |
Fair Value, Measurements, Recurring | Interest rate contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, liabilities | (1.5) | (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 | 0.7 | | |
Derivative financial instruments, liabilities | | (2.9) | (0.9) |
Fair Value, Measurements, Recurring | Level 1 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Non-qualified supplemental postretirement grantor trust investments | 38.4 | 35.6 | 35.8 |
Fair Value, Measurements, Recurring | Level 1 | Commodity contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 58.5 | 27.2 | 29.3 |
Derivative financial instruments, liabilities | (25.3) | (27.7) | (25.6) |
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 | 70 | 76.9 | 10.5 |
Derivative financial instruments, liabilities | (8.9) | (11.4) | (17.3) |
Fair Value, Measurements, Recurring | Level 2 | Foreign currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 18.4 | 12.2 | 11.3 |
Derivative financial instruments, liabilities | (17.9) | (38.2) | (24.3) |
Fair Value, Measurements, Recurring | Level 2 | Interest rate contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, liabilities | (1.5) | (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 | 0.7 | | |
Derivative financial instruments, liabilities | | (2.9) | (0.9) |
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 |