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Debt - 2014 Senior Notes (Details) - USD ($)			3 Months Ended								12 Months Ended
	Mar. 16, 2018	Oct. 06, 2014	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Jul. 21, 2020
Debt
Interest expense			$ (47,500,000)	$ (35,000,000.0)	$ (37,100,000)	$ (40,400,000)	$ (40,200,000)	$ (40,500,000)	$ (41,400,000)	$ (39,400,000)	$ (159,914,000)	$ (161,482,000)	$ (201,023,000)
2014 Senior Notes
Debt
Face amount														$ 2,700,000
Cash tender														$ 639,000,000
2014 Senior Notes | The 2022 Notes
Debt
Face amount		$ 800,000,000
Interest rate (as a percent)		5.75%
Redemption price (in percent)	104.313%
Interest expense	$ 34,500,000
2014 Senior Notes | The 2024 Notes
Debt
Face amount		$ 800,000,000
Interest rate (as a percent)		5.875%
Fair value of debt instrument			$ 863,000,000.0								$ 863,000,000.0
2014 Senior Notes | The 2024 Notes | Prior to July 15 2024
Debt
Redemption price (in percent)		100.00%
2014 Senior Notes | The 2024 Notes | On or after July 15, 2024
Debt
Redemption price (in percent)		100.00%

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Face (par) amount of debt instrument at time of issuance.

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