Fair Value Measurements - Financial Assets and Liabilities that are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions | Jun. 30, 2017 | Sep. 30, 2016 | Jun. 30, 2016 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | $ 51.1 | $ 72.7 | $ 89.1 |
Derivative financial instruments, liabilities | (70.3) | (105.4) | (76) |
Fair Value, Measurements, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Non-qualified supplemental postretirement grantor trust investments | 35.8 | 33 | 32 |
Fair Value, Measurements, Recurring | Commodity contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 39.8 | 54.9 | 70.2 |
Derivative financial instruments, liabilities | (42.9) | (98.6) | (71.4) |
Fair Value, Measurements, Recurring | Foreign currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 11.3 | 17.8 | 18.6 |
Derivative financial instruments, liabilities | (24.3) | (2.4) | (0.8) |
Fair Value, Measurements, Recurring | Interest rate contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, liabilities | (2.2) | (3.9) | (3.8) |
Fair Value, Measurements, Recurring | Cross-currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | | | 0.3 |
Derivative financial instruments, liabilities | (0.9) | (0.5) | |
Fair Value, Measurements, Recurring | Level 1 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Non-qualified supplemental postretirement grantor trust investments | 35.8 | 33 | 32 |
Fair Value, Measurements, Recurring | Level 1 | Commodity contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 29.3 | 28.9 | 39.9 |
Derivative financial instruments, liabilities | (25.6) | (76.8) | (47.1) |
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 | 10.5 | 26 | 30.3 |
Derivative financial instruments, liabilities | (17.3) | (21.8) | (24.3) |
Fair Value, Measurements, Recurring | Level 2 | Foreign currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | 11.3 | 17.8 | 18.6 |
Derivative financial instruments, liabilities | (24.3) | (2.4) | (0.8) |
Fair Value, Measurements, Recurring | Level 2 | Interest rate contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, liabilities | (2.2) | (3.9) | (3.8) |
Fair Value, Measurements, Recurring | Level 2 | Cross-currency contracts | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Derivative financial instruments, assets | | | 0.3 |
Derivative financial instruments, liabilities | (0.9) | (0.5) | |
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 | |