Income Taxes - Reconciliation of Expected 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
Comparison of income tax expense (benefit) and tax computed at the statutory rate
Income taxes at federal statutory rate									$ 446.3	$ 505.0	$ 560.7
Increase (decrease) in tax resulting from:
State income taxes, net of federal benefit									69.1	59.4	42.3
Domestic production activities deduction									0.0	(36.7)	(39.8)
Valuation allowance									(0.2)	(7.3)	0.8
Tax credits									(1.6)	(19.0)	(3.5)
Excess tax benefit from equity compensation									(16.1)	(21.2)	0.0
Tax law change from enactment of Tax Act								$ 108.7	0.0	108.7	0.0
Other									9.2	8.8	3.2
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