Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Mar. 31, 2016	Sep. 30, 2015	Mar. 31, 2015	Sep. 11, 2012
Other Notes Payable | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 44.7		$ 104.2
Other Notes Payable | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	44.7		104.2
Revolving Credit Facility | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	45.0		30.0
Revolving Credit Facility | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	45.0		30.0
Letter of Credit | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	0.0		0.0
Letter of Credit | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	38.9		37.4
Term loan facilities | Term Loan Facilities | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	265.2		280.1
Term loan facilities | Term Loan Facilities | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 266.6		$ 281.3
5.125% Senior Notes due 2023 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument interest rate (percent)	5.125%		5.125%			5.125%
5.125% Senior Notes due 2023 | Senior Notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 297.3		$ 296.6
5.125% Senior Notes due 2023 | Senior Notes | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 306.6		$ 298.5
5.75% Senior Notes due 2024 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument interest rate (percent)	5.75%		5.75%				5.75%
5.75% Senior Notes due 2024 | Senior Notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 697.3		$ 1,194.7
5.75% Senior Notes due 2024 | Senior Notes | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 708.8		$ 1,200.0
5.75% Senior Notes due 2025 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument interest rate (percent)	5.75%		5.75%	5.75%	5.75%
5.75% Senior Notes due 2025 | Senior Notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 345.6		$ 345.0
5.75% Senior Notes due 2025 | Senior Notes | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 369.7		339.5
4.50% Senior Notes due 2028 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument interest rate (percent)	4.50%	4.50%
4.50% Senior Notes due 2028 | Senior Notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 491.7		0.0
4.50% Senior Notes due 2028 | Senior Notes | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 519.4		0.0
4.75% Senior Notes due 2030 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument interest rate (percent)	4.75%	4.75%
4.75% Senior Notes due 2030 | Senior Notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 491.7		0.0
4.75% Senior Notes due 2030 | Senior Notes | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amounts and estimated fair values of financial instruments	$ 520.0		$ 0.0