Income Taxes - Income Tax Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Current tax expense (benefit):
Federal									$ 373.2	$ 425.6	$ 376.0
State									53.6	27.3	15.9
Total current tax expense (benefit)									426.8	452.9	391.9
Deferred tax expense:
Federal									158.7	87.9	47.6
State									12.2	22.9	27.7
Total deferred tax expense (benefit)									170.9	110.8	75.3
Total income tax expense (benefit)	$ 138.8	$ 162.5	$ 94.0	$ 202.4	$ 172.3	$ 155.5	$ 124.7	$ 111.2	$ 597.7	$ 563.7	$ 467.2