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Borrowing Arrangements (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Mar. 31, 2019 | Jun. 30, 2019 | Jun. 30, 2018 | Sep. 30, 2018 | Jun. 12, 2018 | Sep. 30, 2017
Agreement termination date |  |  |  | Jul. 13, 2021 |  |  |  |
Cash and cash equivalents | $ 140.7 | $ 331.5 |  | $ 140.7 | $ 331.5 | $ 347.1 |  | $ 361.7
Future maturities of outstanding borrowings |  |  |  |  |  |  |  |
Loss on early extinguishment of debt | 0.0 | 6.2 |  | 0.0 | 6.2 |  |  |
Payments of Debt Issuance Costs |  |  |  | 0.0 | $ 6.6 |  |  |
Interest Costs Capitalized |  |  |  | 1.2 |  |  |  |
Domestic Line of Credit [Member] |  |  |  |  |  |  |  |
Revolving credit facility amount |  |  | $ (175.0) |  |  |  |  |
Potential increase size of the credit facility by an additional amount |  |  | $ 150.0 |  |  |  |  |
Line of Credit Facility, Interest Rate at Period End |  |  | 12500.00% |  |  |  |  |
Aggregate commitments availability |  |  | $ 17.5 |  |  |  |  |
Aggregate commitments availability, percentage |  |  | 10.00% |  |  |  |  |
Outstanding letter of credit accrued fees and expenses | 16.4 |  |  | 16.4 |  |  |  |
Excess availability reduced by outstanding borrowings, outstanding letters of credit and accrued fees and expenses | 136.4 |  |  | 136.4 |  |  |  |
Unsecured Debt [Member] |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  |  |  | $ 450.0 |
Future maturities of outstanding borrowings |  |  |  |  |  |  |  |
Payments of Debt Issuance Costs |  | $ 6.6 |  |  |  |  |  |
Swing Line Loans [Member] |  |  |  |  |  |  |  |
Revolving credit facility amount |  |  | $ (25.0) |  |  |  |  |
Letters Of Credit Outstanding [Member] |  |  |  |  |  |  |  |
Revolving credit facility amount |  |  | $ (60.0) |  |  |  |  |
Bonds [Member] |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  | 5.50% |  |  |  |  |
Future maturities of outstanding borrowings |  |  |  |  |  |  |  |
Financial Liabilities Fair Value Disclosure | $ 465.8 |  |  | $ 465.8 |  |  |  |
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] | 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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