Fair Value Measurement (Tables)

6 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Financial Assets and Financial Liabilities that are Measured at Fair Value on a Recurring Basis

The following table presents on a gross basis our financial assets and liabilities, including both current and noncurrent portions, that are measured at fair value on a recurring basis within the fair value hierarchy, as of March 31, 2018, September 30, 2017 and March 31, 2017:

	Asset (Liability)
	Level 1			Level 2			Level 3		Total
March 31, 2018:
Derivative instruments:
Assets:
Commodity contracts	$	39.6		$	27.0		$	—	$	66.6
Foreign currency contracts	$	—		$	13.9		$	—	$	13.9
Liabilities:
Commodity contracts	$	(24.3	)	$	(13.5	)	$	—	$	(37.8	)
Foreign currency contracts	$	—		$	(43.5	)	$	—	$	(43.5	)
Interest rate contracts	$	—		$	(1.9	)	$	—	$	(1.9	)
Cross-currency contracts	$	—		$	(2.2	)	$	—	$	(2.2	)
Non-qualified supplemental postretirement grantor trust investments (a)	$	37.9		$	—		$	—	$	37.9
September 30, 2017:
Derivative instruments:
Assets:
Commodity contracts	$	27.2		$	76.9		$	—	$	104.1
Foreign currency contracts	$	—		$	12.2		$	—	$	12.2
Liabilities:
Commodity contracts	$	(27.7	)	$	(11.4	)	$	—	$	(39.1	)
Foreign currency contracts	$	—		$	(38.2	)	$	—	$	(38.2	)
Interest rate contracts	$	—		$	(2.3	)	$	—	$	(2.3	)
Cross-currency contracts	$	—		$	(2.9	)	$	—	$	(2.9	)
Non-qualified supplemental postretirement grantor trust investments (a)	$	35.6		$	—		$	—	$	35.6
March 31, 2017:
Derivative instruments:
Assets:
Commodity contracts	$	56.0		$	18.0		$	—	$	74.0
Foreign currency contracts	$	—		$	15.6		$	—	$	15.6
Cross-currency contracts	$	—		$	3.0		$	—	$	3.0
Liabilities:
Commodity contracts	$	(28.0	)	$	(11.0	)	$	—	$	(39.0	)
Foreign currency contracts	$	—		$	(1.5	)	$	—	$	(1.5	)
Interest rate contracts	$	—		$	(2.2	)	$	—	$	(2.2	)
Non-qualified supplemental postretirement grantor trust investments (a)	$	35.2		$	—		$	—	$	35.2

(a) Consists primarily of mutual fund investments held in grantor trusts associated with non-qualified supplemental retirement plans.

Schedule of Carrying Amount and Estimated Fair Value of Long-term Debt

The carrying amount and estimated fair value of our long-term debt (including current maturities but excluding unamortized debt issuance costs) at March 31, 2018, September 30, 2017 and March 31, 2017 were as follows:

	March 31, 2018		September 30, 2017		March 31, 2017
Carrying amount	$	4,316.4	$	4,211.9	$	4,238.9
Estimated fair value	$	4,287.3	$	4,346.8	$	4,255.0