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Debt - 2014 Senior Notes (Details) - USD ($)			3 Months Ended								12 Months Ended
	Mar. 16, 2018	Oct. 06, 2014	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Debt
Interest expense			$ (56,400,000)	$ (55,700,000)	$ (57,900,000)	$ (56,000,000.0)	$ (55,500,000)	$ (55,300,000)	$ (100,500,000)	$ (56,200,000)	$ (225,994,000)	$ (267,519,000)	$ (231,310,000)
2014 Senior Notes | The 2022 Notes
Debt
Principal amount		$ 800,000,000
Interest rate (as a percent)		5.75%
Redemption price (in percent)	104.313%
2014 Senior Notes | The 2024 Notes
Debt
Principal amount		$ 800,000,000
Interest rate (as a percent)		5.875%
Fair value of debt instrument			$ 866,000,000.0								$ 866,000,000.0
Interest expense	$ 34,500,000
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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