Income Taxes - Reconciliation of Expected Income Tax Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Comparison of income tax expense (benefit) and tax computed at the statutory rate
Income taxes at federal statutory rate									$ 473.7	$ 393.2	$ 285.0
Increase (decrease) in tax resulting from:
State income taxes, net of federal benefit									38.6	37.0	24.9
Domestic production activities deduction									(36.3)	(35.7)	(22.4)
Uncertain tax positions, net of deferred tax									0.0	0.0	(6.4)
Valuation allowance									0.2	(21.0)	0.1
Tax credits									(15.9)	(2.2)	(0.9)
Other									6.9	1.4	0.4
Total income tax expense (benefit)	$ 149.4	$ 128.8	$ 105.4	$ 83.6	$ 99.9	$ 112.4	$ 82.2	$ 78.2	$ 467.2	$ 372.7	$ 280.7