Debt Obligations - Senior Credit Agreement (Details)	9 Months Ended
Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
2016 Credit Facilities
Debt Instrument [Line Items]
Principal amount of line of credit	$ 1,325,000,000
2016 Credit Facilities \| LIBOR \| Minimum
Debt Instrument [Line Items]
Spread on variable rates	1.50%
2016 Credit Facilities \| LIBOR \| Maximum
Debt Instrument [Line Items]
Spread on variable rates	2.50%
2016 Credit Facilities \| Base Rate \| Minimum
Debt Instrument [Line Items]
Spread on variable rates	0.50%
2016 Credit Facilities \| Base Rate \| Maximum
Debt Instrument [Line Items]
Spread on variable rates	1.50%
Line of Credit \| 2016 Credit Facilities
Debt Instrument [Line Items]
Maximum consolidated interest coverage ratio	4.5
Minimum consolidated interest coverage ratio	3.0
Secured Debt \| Term Loans
Debt Instrument [Line Items]
Principal amount of line of credit	$ 875,000,000
Secured Debt \| Line of Credit \| Term Loans
Debt Instrument [Line Items]
Term of debt	5 years
Outstanding principal balance of debt	$ 274,000,000	$ 285,000,000
Quarterly installments due in fiscal 2018	4,000,000
Quarterly installments due in fiscal 2019	8,000,000
Quarterly installments due in fiscal 2020	8,000,000
Quarterly installments due in fiscal 2021	15,000,000
Revolver
Debt Instrument [Line Items]
Principal amount of line of credit	450,000,000
Borrowings from revolving credit facility	70,000,000
Repayments of revolving credit facility	26,000,000
Total borrowing capacity remaining	$ 395,000,000
Revolver \| Line of Credit
Debt Instrument [Line Items]
Term of debt	5 years
Long-term debt outstanding amount	$ 44,000,000	0
Letter of Credit
Debt Instrument [Line Items]
Principal amount of line of credit	100,000,000
Line of Credit \| Revolver
Debt Instrument [Line Items]
Long-term debt outstanding amount	$ 44,000,000	$ 0

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of cash inflow from contractual arrangement with the lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

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Amount of cash outflow for payment of an obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

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Debt Instrument, Covenant, Interest Coverage Ratio, Minimum

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