Income Taxes - Income Tax Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Current tax expense (benefit):
Federal									$ 407.3	$ 373.2	$ 425.6
State									79.3	53.6	27.3
Total current tax expense (benefit)									486.6	426.8	452.9
Deferred tax expense:
Federal									13.9	158.7	87.9
State									6.2	12.2	22.9
Total deferred tax expense (benefit)									20.1	170.9	110.8
Total income tax expense (benefit)	$ 156.2	$ 153.1	$ 108.4	$ 89.0	$ 138.8	$ 162.5	$ 94.0	$ 202.4	$ 506.7	$ 597.7	$ 563.7