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Debt - Effective Interest Rate (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Mar. 31, 2018 | Mar. 31, 2017
Debt |  |  |  |
Effective interest rate including effects of interest rate swap agreements |  |  | 4.7% | 4.2%
Amortization of deferred debt issuance costs | $ 9.8 | $ 8.7 | $ 12.6 | $ 11.5
The 2022 Notes |  |  |  |
Debt |  |  |  |
Prepayment premium redemption | $ 34.5 |  | $ 34.5 |

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