Income Taxes - 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
Current tax expense (benefit):
Federal									$ 484.0	$ 407.3	$ 373.2
State									104.4	79.3	53.6
Total current tax expense (benefit)									588.4	486.6	426.8
Deferred tax expense:
Federal									20.0	13.9	158.7
State									(5.9)	6.2	12.2
Total deferred tax expense (benefit)									14.1	20.1	170.9
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
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount									$ (11.2)	$ (0.2)	$ (7.3)