Income Taxes - Schedule of Components of Provision for (Benefit from) Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2021
Current:
Federal	$ 168.3	$ 268.4	$ 98.1	$ (94.7)
Foreign	1.0	29.9	42.0	8.5
State	11.4	52.1	10.9	22.8
Total current	180.7	350.4	151.0	(63.4)
Deferred:
Federal	(166.4)	(126.0)	(53.3)	119.3
Foreign	(0.5)	(8.0)	(1.4)	(5.6)
State	(12.6)	(26.4)	1.2	(13.9)
Total deferred	(179.5)	(160.4)	(53.5)	99.8
Provision for income taxes	1.2	190.0	97.5	36.4
Allocated to Other Comprehensive Income (Loss)
Deferred federal, state and foreign	$ (2.8)	$ 1.8	$ (19.2)	$ (20.0)