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Borrowing Arrangements (Narrative) (Details) - USD ($) $ in Thousands | 6 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2018 | Sep. 30, 2017 | Mar. 31, 2017 | Sep. 30, 2016
Cash and cash equivalents | $ 323,900 | $ 361,700 | $ 328,300 | $ 195,000
Future maturities of outstanding borrowings |  |  |  |
Debt Instrument, Unamortized Discount | 1,300 |  |  |
Domestic Line of Credit [Member] |  |  |  |
Revolving credit facility amount | (225,000) |  |  |
Potential increase size of the credit facility by an additional amount | $ 150,000 |  |  |
Line of Credit Facility, Interest Rate at Period End | 12500.00% |  |  |
Agreement termination date | Jul. 13, 2021 |  |  |
Aggregate commitments availability | $ 17,500 |  |  |
Aggregate commitments availability, percentage | 10.00% |  |  |
Outstanding letter of credit accrued fees and expenses | $ 18,500 |  |  |
Excess availability reduced by outstanding borrowings, outstanding letters of credit and accrued fees and expenses | 133,500 |  |  |
Swing Line Loans [Member] |  |  |  |
Revolving credit facility amount | (25,000) |  |  |
Letters Of Credit Outstanding [Member] |  |  |  |
Revolving credit facility amount | (60,000) |  |  |
Secured Debt [Member] |  |  |  |
Long-term Debt, Gross | $ 500,000 |  |  |
Debt Instrument, Basis Spread on Variable Rate | 25000.00% |  |  |
Debt Instrument, Interest Rate, Effective Percentage | 4.78% |  |  |
Debt Instrument, Periodic Payment, Principal | $ 1,225 |  |  |
Agreement termination date | Nov. 25, 2021 |  |  |
Future maturities of outstanding borrowings |  |  |  |
Financial Liabilities Fair Value Disclosure | $ 488,400 |  |  |
Minimum [Member] | Domestic Line of Credit [Member] |  |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage | 2500.00% |  |  |
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Long-term Debt [Member] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 2500.00% |  |  |
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Domestic Line of Credit [Member] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 12500.00% |  |  |
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Secured Debt [Member] |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum (Deprecated 2016-01-31) | 0.75% |  |  |
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | Domestic Line of Credit [Member] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 15000.00% |  |  |
Base Rate [Member] | Maximum [Member] | Domestic Line of Credit [Member] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 5000.00% |  |  |

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