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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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