Fair Value (Tables)

6 Months Ended

Mar. 31, 2023

Fair Value Disclosures [Abstract]

Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis

Assets and liabilities recorded at fair value on a recurring basis consisted of the following (in millions):         

	As of
	March 31, 2023								September 30, 2022
			Fair Value Measurements								Fair Value Measurements
	Total		Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3
Assets
Cash and cash equivalents (1)	$	832.6	$	624.9	$	207.7	$	—	$	566.0	$	565.7	$	0.3	$	—
U.S. Treasury and government securities	164.5		8.2		156.3		—		13.6		3.6		10.0		—
Corporate bonds and notes	63.7		—		63.7		—		0.2		—		0.2		—
Municipal bonds	0.6		—		0.6		—		7.0		—		7.0		—
Total assets at fair value	$	1,061.4	$	633.1	$	428.3	$	—	$	586.8	$	569.3	$	17.5	$	—

Schedule of Carrying Values and Estimated Fair Values of Debt Instruments

The carrying amount and estimated fair value of debt consists of the following (in millions):

	As of
	March 31, 2023				September 30, 2022
	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
0.90% Senior Notes due 2023	$	499.8	$	495.6	$	499.2	$	488.5
1.80% Senior Notes due 2026	497.3		452.9		496.8		431.2
3.00% Senior Notes due 2031	494.8		420.5		494.5		377.6
Total debt under Senior Notes	$	1,491.9	$	1,369.0	$	1,490.5	$	1,297.3