Debt - Text (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Oct. 31, 2025	Sep. 30, 2025	Mar. 31, 2025	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Oct. 30, 2025	Apr. 30, 2020
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 640.0		$ 640.0
Line of Credit Facility, Maximum Borrowing Capacity		1,000.0		1,000.0
Letter of Credit, Maximum Borrowing Capacity		$ 100.0		$ 100.0
Letter of Credit, Maximum Borrowing Capacity, Percentage of Revolving Credit Commitment		50.00%		50.00%
Long-term Line of Credit		$ 0.0		$ 0.0	$ 0.0
Letters of Credit Outstanding, Amount		51.1		51.1
Line of Credit Facility, Remaining Borrowing Capacity		588.9		588.9
Debt Repurchase Program, Authorized Amount		30.0		30.0				$ 30.0
Debt		802.7		802.7	706.4
Loss on extinguishment of debt				1.2	0.0	$ 0.0
Debt Redemption, Total Amount		71.6
Subsequent Event [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity							$ 665.0
Credit Facility Maturing December 18, ,2029
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		575.0		575.0
Credit Facility Maturing December 18, ,2029 | Subsequent Event [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity							$ 600.0
Line of Credit Facility, Increase (Decrease), Net	$ 25.0
Credit Facility Maturing October 28, 2026
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 65.0		$ 65.0
Senior Notes 3.85% [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.85%		3.85%
Debt		$ 0.0		$ 0.0	398.4
Debt Instrument, Repurchase Amount		329.4		329.4
Debt Instrument, Repurchase, Accrued and Unpaid Interest		1.0	$ 4.2
Debt Instrument, Total Repurchase Price		333.4		333.4
Debt Instrument, Repurchased Face Amount		400.0		400.0
Loss on extinguishment of debt				$ 1.2
Debt Redemption, Principal Amount Redeemed		$ 70.6
Senior Notes 5.0% [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.00%		5.00%
Debt Instrument, Redemption Price, Percentage				100.833%
Debt Instrument, Interest Rate, Effective Percentage		5.20%		5.20%
Debt		$ 298.7		$ 298.7	298.1
Debt Instrument, Face Amount		$ 300.0		$ 300.0
Senior Notes 6.5%
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		6.50%		6.50%
Debt Instrument, Interest Rate, Effective Percentage		6.70%		6.70%
Debt		$ 494.1		$ 494.1	0.0
Debt Instrument, Issued, Principal				$ 500.0
Senior Notes 6.5% | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage				106.50%
Debt Instrument, Redemption Period, End Date				Mar. 15, 2028
Debt Instrument, Redemption Price, Maximum Percentage of Principal Amount Redeemed				40.00%
Senior Notes 6.5% | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage				103.25%
Debt Instrument, Redemption Period, End Date				Mar. 15, 2030
Debt Instrument, Redemption Price, Maximum Percentage of Principal Amount Redeemed				100.00%
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Net		$ 7.2		$ 7.2	3.5
Secured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.00%		4.00%
Debt		$ 9.9		$ 9.9	$ 9.9