Income Taxes - Reconciliation of Expected Income Tax Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Comparison of income tax expense (benefit) and tax computed at the statutory rate
Income taxes at federal statutory rate									$ 626.4	$ 446.3	$ 505.0
Increase (decrease) in tax resulting from:
State income taxes, net of federal benefit									79.1	69.1	59.4
Domestic production activities deduction									0.0	0.0	(36.7)
Valuation allowance									(11.2)	(0.2)	(7.3)
Tax credits									(93.4)	(1.6)	(19.0)
Excess tax benefit from equity compensation									(22.3)	(16.1)	(21.2)
Tax law change from enactment of Tax Act									0.0	0.0	108.7
Effective Income Tax Rate Reconciliation, Tax Contingency, Amount									8.9	0.0	0.0
Other									15.0	9.2	8.8
Total income tax expense (benefit)	$ 224.9	$ 149.5	$ 137.3	$ 90.8	$ 156.2	$ 153.1	$ 108.4	$ 89.0	$ 602.5	$ 506.7	$ 597.7