Financial Instruments and Risk Management - Narrative (Details)	3 Months Ended			9 Months Ended
	Jun. 30, 2020 USD ($) derivative_instrument Contract	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2020 USD ($) derivative_instrument Contract	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Apr. 01, 2019 USD ($)	Feb. 28, 2018 USD ($)	Jun. 30, 2017 USD ($)
Derivative [Line Items]
Unrecognized pre-tax loss			$ (4,700,000)	$ 8,900,000
Portion or pre-tax gain included in AOCI expected to be included in earnings	$ (1,000,000.0)			(1,000,000.0)
Gain (Loss) Recognized in Income	0	$ (900,000)		(2,200,000)	$ 8,100,000
Interest Rate Contracts
Derivative [Line Items]
Notional value	150,000,000.0			150,000,000.0			$ 50,000,000.0	$ 400,000,000.0
Forward interest rate								2.47%
Line of Credit | Senior Secured Term Loan B Facility, net of discount, due 2022
Derivative [Line Items]
Face amount of debt	855,700,000			855,700,000
Estimate of Fair Value
Derivative [Line Items]
Fair market value of fixed rate long-term debt	2,722,300,000			2,722,300,000		$ 2,474,700,000
Reported Value Measurement
Derivative [Line Items]
Fair market value of fixed rate long-term debt	2,680,200,000			2,680,200,000		2,408,400,000
Senior Secured Term Loan B Facility due 2022 | Senior Secured Term Loan B Facility, net of discount, due 2022 | Interest Rate Contracts
Derivative [Line Items]
Variable rate debt hedged									$ 200,000,000.0
Fixed interest rate									2.03%
Designated as Hedging Instrument | Cash Flow Hedging
Derivative [Line Items]
Derivatives - Foreign Currency contracts	(13,600,000)			(13,600,000)		(1,200,000)
Designated as Hedging Instrument | Cash Flow Hedging | Zinc contracts
Derivative [Line Items]
Derivatives - Foreign Currency contracts	$ (3,500,000)			$ (3,500,000)		(1,000,000.0)
Derivative, Number of Open Contracts | Contract	19			19
Notional value	$ 51,000,000			$ 51,000,000
Designated as Hedging Instrument | Cash Flow Hedging | Foreign currency contracts
Derivative [Line Items]
Derivatives - Foreign Currency contracts	(1,200,000)			(1,200,000)		4,500,000
Designated as Hedging Instrument | Cash Flow Hedging | Foreign Exchange Forward
Derivative [Line Items]
Notional value	$ 141,000,000			$ 141,000,000
Number of open contracts | derivative_instrument	64			64
Not Designated as Hedging Instrument | Foreign currency contracts
Derivative [Line Items]
Derivatives - Foreign Currency contracts	$ (500,000)			$ (500,000)		$ 4,300,000
Gain (Loss) Recognized in Income	(600,000)	$ (1,300,000)		(2,200,000)	$ (200,000)
Not Designated as Hedging Instrument | Foreign Exchange Forward
Derivative [Line Items]
Notional value	$ 80,000,000			$ 80,000,000
Number of open contracts | derivative_instrument	8			8