Debt - Text (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024	Sep. 30, 2024
Debt Instrument [Line Items]
Long-term Line of Credit	$ 0.0		$ 0.0		$ 0.0
Debt	872.5		872.5		706.4
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%
Letters of Credit Outstanding, Amount	$ 22.3		$ 22.3
Line of Credit Facility, Remaining Borrowing Capacity	617.7		617.7
Debt Repurchase Program, Authorized Amount	30.0		30.0
Debt Repurchase Authorization Remaining	30.0		30.0
Gain (Loss) on Repurchase of Debt Instrument	(1.1)	$ 0.0	(1.1)	$ 0.0
Credit Facility Maturing December 18, 2029
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	575.0		575.0
Credit Facility Maturing 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	$ 70.4		$ 70.4		398.4
Debt Instrument, Face Amount	$ 70.6		$ 70.6
Debt Instrument, Frequency of Periodic Payment			semiannually
Debt Instrument, Interest Rate, Effective Percentage	4.10%		4.10%
Debt Instrument, Redemption Price	$ 333.4		$ 333.4
Debt Instrument, Redemption, Accrued and Unpaid Interest	4.2
Debt Instrument, Repurchased Face Amount	400.0		400.0
Debt Instrument, Repurchase Amount	$ 329.4		$ 329.4
Debt Instrument, Maturity Date			May 15, 2026
Senior Notes 3.85% [Member] | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage			100.963%
Debt Instrument, Redemption Period, End Date			May 15, 2025
Senior Notes 5.0% [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.00%		5.00%
Debt	$ 298.4		$ 298.4		298.1
Debt Instrument, Face Amount	$ 300.0		$ 300.0
Debt Instrument, Redemption Price, Percentage			100.833%
Debt Instrument, Frequency of Periodic Payment			semiannually
Debt Instrument, Interest Rate, Effective Percentage	5.20%		5.20%
Debt Instrument, Maturity Date			Mar. 01, 2028
Senior Notes 5.0% [Member] | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Debt Instrument, Redemption Period, End Date			Mar. 01, 2026
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ 8.0		$ 8.0		3.5
Unsecured Debt
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.00%		4.00%
Debt	$ 9.9		$ 9.9		9.9
Senior Notes 6.50%
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.50%		6.50%
Debt	$ 493.8		$ 493.8		$ 0.0
Debt Instrument, Frequency of Periodic Payment			semiannually
Debt Instrument, Interest Rate, Effective Percentage	6.70%		6.70%
Debt Instrument, Issued, Principal	$ 500.0
Debt Instrument, Maturity Date			Mar. 15, 2033
Senior Notes 6.50% | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage			106.50%
Debt Instrument, Redemption Price, Maximum Percentage of Principal Amount Redeemed			40.00%
Debt Instrument, Redemption Period, End Date			Mar. 15, 2028
Senior Notes 6.50% | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage			103.25%
Debt Instrument, Redemption Price, Maximum Percentage of Principal Amount Redeemed			100.00%
Debt Instrument, Redemption Period, End Date			Mar. 15, 2030