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Debt - 2014 Senior Notes (Details) - USD ($) |  |  | 3 Months Ended | 6 Months Ended
--- | --- | --- | --- | ---
Mar. 16, 2018 | Oct. 06, 2014 | Mar. 31, 2018 | Mar. 31, 2018
The 2022 Notes |  |  |  |
Debt |  |  |  |
Prepayment premium redemption |  |  | $ 34,500,000 | $ 34,500,000
2014 Senior Notes | The 2022 Notes |  |  |  |
Debt |  |  |  |
Principal amount |  | $ 800,000,000 |  |
Interest rate (as a percent) |  | 5.75% |  |
Redemption price equal to principal amount (in percent) | 104.313% |  |  |
Prepayment premium redemption |  |  | 34,500,000 | 34,500,000
2014 Senior Notes | The 2024 Notes |  |  |  |
Debt |  |  |  |
Principal amount |  | $ 800,000,000 |  |
Interest rate (as a percent) |  | 5.875% |  |
Fair value of debt instrument |  |  | $ 836,000,000 | $ 836,000,000
2014 Senior Notes | The 2024 Notes | Prior to July 15 2024 |  |  |  |
Debt |  |  |  |
Redemption price equal to principal amount (in percent) |  | 100.00% |  |
2014 Senior Notes | The 2024 Notes | On or after July 15, 2024 |  |  |  |
Debt |  |  |  |
Redemption price equal to principal amount (in percent) |  | 100.00% |  |

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Percentage price of original principal amount of debt at which debt can be redeemed by the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph e -URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823 + Details
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