Income Taxes - Reconciliation of Expected Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Comparison of income tax expense (benefit) and tax computed at the statutory rate
Income taxes at federal statutory rate	$ 1,124.8	$ 626.4	$ 446.3
Increase (decrease) in tax resulting from:
State income taxes, net of federal benefit	166.9	79.1	69.1
Valuation allowance	(3.3)	(11.2)	(0.2)
Tax credits	(116.3)	(93.4)	(1.6)
Excess tax benefit from equity compensation	(38.4)	(22.3)	(16.1)
Effective Income Tax Rate Reconciliation, Tax Contingency, Amount	(6.0)	8.9	0.0
Other	37.4	15.0	9.2
Total income tax expense (benefit)	$ 1,165.1	$ 602.5	$ 506.7

Increase decrease in tax resulting from.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax credits. Including, but not limited to, research credit, foreign tax credit, investment tax credit, and other tax credits.

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