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Derivative Financial Instruments and Fair Value Measurements - Cash Flow Hedges (Details) - Designated as Hedging Instrument - Cash Flow Hedging $ in Millions, $ in Millions | Mar. 31, 2018 AUD ($) | Mar. 31, 2018 USD ($) | Sep. 30, 2017 AUD ($) | Sep. 30, 2017 USD ($)
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Interest Rate Swap June 2018 | USD |  |  |  |
Derivative financial instruments |  |  |  |
Notional Amount |  | $ 300.0 |  | $ 300.0
Fixed Rate (as a percent) | 1.63% | 1.63% | 1.63% | 1.63%
Interest Rate Swap September 2018 | USD |  |  |  |
Derivative financial instruments |  |  |  |
Notional Amount |  | $ 300.0 |  | $ 300.0
Fixed Rate (as a percent) | 1.54% | 1.54% | 1.54% | 1.54%
Interest Rate Swap February 2021 | AUD |  |  |  |
Derivative financial instruments |  |  |  |
Notional Amount |  | $ 200.0 |  |
Fixed Rate (as a percent) | 2.19% | 2.19% |  |
Interest Rate Swap September 2022 | CAD |  |  |  |
Derivative financial instruments |  |  |  |
Notional Amount |  | $ 400.0 |  |
Fixed Rate (as a percent) | 2.49% | 2.49% |  |
Interest Rate Swap February 2023 | USD |  |  |  |
Derivative financial instruments |  |  |  |
Notional Amount |  | $ 200.0 |  |
Fixed Rate (as a percent) | 2.60% | 2.60% |  |
Foreign currency contracts |  |  |  |
Derivative financial instruments |  |  |  |
Notional principal of derivative exchange contracts | $ 2.9 | $ 2.3 | $ 15.1 | $ 11.3

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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