Income Taxes - 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
Current tax expense (benefit):
Federal									$ 376.0	$ 356.4	$ 253.6
State									15.9	13.2	9.1
Total current tax expense (benefit)									391.9	369.6	262.7
Deferred tax expense:
Federal									47.6	(3.1)	2.9
State									27.7	6.2	15.1
Total deferred tax expense (benefit)									75.3	3.1	18.0
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