Fair Value Measurement - Additional Information (Detail) - Fair Value, Inputs, Level 2 [Member] $ in Millions	12 Months Ended
	Sep. 30, 2020 USD ($)
2026 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Debt instrument maturity year	2026
2025 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Debt instrument maturity year	2025
Carrying Value [Member] | 2026 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	$ 300.0
Carrying Value [Member] | 2025 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	1,300.0
Fair Value [Member] | 2026 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	309.0
Fair Value [Member] | 2025 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	$ 1,280.0