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Income Taxes - Income Tax Expense (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Current tax expense (benefit):                      
Federal                 $ 425.6 $ 376.0 $ 356.4
State                 27.3 15.9 13.2
Total current tax expense (benefit)                 452.9 391.9 369.6
Deferred tax expense:                      
Federal                 87.9 47.6 (3.1)
State                 22.9 27.7 6.2
Total deferred tax expense (benefit)                 110.8 75.3 3.1
Total income tax expense (benefit) $ 172.3 $ 155.5 $ 124.7 $ 111.2 $ 149.4 $ 128.8 $ 105.4 $ 83.6 $ 563.7 $ 467.2 $ 372.7